It might be the end. It is certainly the beginning of the end.

Across Egypt, tens of thousands of Arabs braved tear gas, water cannon, stun grenades and live fire yesterday to demand the removal of Hosni Mubarak after more than 30 years of dictatorship.

And as Cairo lay drenched under clouds of tear gas from thousands of canisters fired into dense crowds by riot police, it looked as if his rule was nearing its finish. None of us on the streets of Cairo yesterday even knew where Mubarak was. And I didn't find anyone who cared.

They were brave, largely peaceful, these tens of thousands, but the shocking behaviour of Mubarak's plainclothes battagi – the word does literally mean "thugs" in Arabic – who beat, bashed and assaulted demonstrators while the cops watched and did nothing, was a disgrace. These men, many of them ex-policemen who are drug addicts, were last night the front line of the Egyptian state. The true representatives of Hosni Mubarak as uniformed cops showered gas on to the crowds.

At one point last night, gas canisters were streaming smoke across the waters of the Nile as riot police and protesters fought on the great river bridges. It was incredible, a risen people who would no longer take violence and brutality and prison as their lot in the largest Arab nation. And the police themselves might be cracking: "What can we do?" one of the riot cops asked us. "We have orders. Do you think we want to do this? This country is going downhill." The government imposed a curfew last night as protesters knelt in prayer in front of police.

How does one describe a day that may prove to be so giant a page in Egypt's history? Maybe reporters should abandon their analyses and just tell the tale of what happened from morning to night in one of the world's most ancient cities. So here it is, the story from my notes, scribbled amid a defiant people in the face of thousands of plainclothes and uniformed police.

It began at the Istikama mosque on Giza Square: a grim thoroughfare of gaunt concrete apartment blocks and a line of riot police that stretched as far as the Nile. We all knew that Mohamed El-Baradei would be there for midday prayers and, at first, the crowd seemed small. The cops smoked cigarettes. If this was the end of the reign of Mubarak, it was a pretty unimpressive start.

But then, no sooner had the last prayers been uttered than the crowd of worshippers, perched above the highway, turned towards the police. "Mubarak, Mubarak," they shouted. "Saudi Arabia is waiting for you." That's when the water cannon were turned on the crowd – the police had every intention of fighting them even though not a stone had been thrown. The water smashed into the crowd and then the hoses were pointed directly at El-Baradei, who reeled back, drenched.

He had returned from Vienna a few hours earlier and few Egyptians think he will run Egypt – he claims to want to be a negotiator – but this was a disgrace. Egypt's most honoured politician, a Nobel-prize winner who had held the post of the UN's top nuclear inspector, was drenched like a street urchin. That's what Mubarak thought of him, I suppose: just another trouble maker with a "hidden agenda" – that really is the language the Egyptian government is using right now.

And then the tear gas burst over the crowds. Perhaps there were a few thousand now, but as I walked beside them, something remarkable happened. From apartment blocks and dingy alleyways, from neighbouring streets, hundreds and then thousands of Egyptians swarmed on to the highway leading to Tahrir Square. This is the one tactic the police had decided to prevent. To have Mubarak's detractors in the very centre of Cairo would suggest that his rule was already over. The government had already cut the internet – slicing off Egypt from the rest of the world – and killed all of the mobile phone signals. It made no difference.

"We want the regime to fall," the crowds screamed. Not perhaps the most memorable cry of revolution but they shouted it again and again until they drowned out the pop of tear gas grenades. From all over Cairo they surged into the city, middle class youngsters from Gazira, the poor from the slums of Beaulak al-Daqrour, marching steadily across the Nile bridges like an army – which, I guess, was what they were.

Still the gas grenades showered over them. Coughing and retching, they marched on. Many held their coats over their mouths or queued at a lemon shop where the owner squeezed fresh fruit into their mouths. Lemon juice – an antidote to tear gas – poured across the pavement into the gutter.

This was Cairo, of course, but these protests were taking place all over Egypt, not least in Suez, where at least six Egyptians have so far been killed. The demonstrations began not just at mosques but at Coptic churches. "I am a Christian, but I am an Egyptian first," a man called Mina told me. "I want Mubarak to go." And that is when the first bataggi arrived, pushing to the front of the police ranks in order to attack the protesters. They had metal rods and police truncheons – from where? – and sharpened sticks, and could be prosecuted for serious crimes if Mubarak's regime falls. They were vicious. One man whipped a youth over the back with a long yellow cable. He howled with pain. Across the city, the cops stood in ranks, legions of them, the sun glinting on their visors. The crowd were supposed to be afraid, but the police looked ugly, like hooded birds. Then the protesters reached the east bank of the Nile.

A few tourists found themselves caught up in this spectacle – I saw three middle-aged ladies on one of the Nile bridges (Cairo's hotels had not, of course, told their guests what was happening) – but the police decided that they would hold the east end of the flyover. They opened their ranks again and sent the thugs in to beat the leading protesters. And this was the moment the tear gassing began in earnest, hundreds upon hundreds of canisters raining on to the crowds who marched from all roads into the city. It stung our eyes and made us cough until we were gasping. Men were being sick beside sealed shop fronts.

Fires appear to have broken out last night near the Mubarak's rubber-stamp NDP headquarters. A curfew was imposed and first reports spoke of troops in the city, an ominous sign that the police had lost control. We took refuge in the old Café Riche off Telaat Harb Square, a tiny restaurant and bar of blue-robed waiters; and there, sipping his coffee, was the great Egyptian writer Ibrahim Abdul Meguid, right in front of us. It was like bumping into Tolstoy taking lunch amid the Russian revolution. "There has been no reaction from Mubarak!" he exalted. "It is as if nothing has happened! But they will do it – the people will do it!" The guests sat choking from the gas. It was one of those memorable scenes that occur in movies rather than real life.

And there was an old man on the pavement, one hand over his stinging eyes. Retired Colonel Weaam Salim of the Egyptian army, wearing his medal ribbons from the 1967 war with Israel – which Egypt lost – and the 1973 war, which the colonel thought Egypt had won. "I am leaving the ranks of veteran soldiers," he told me. "I am joining the protesters." And what of the army? Throughout the day we had not seen them. Their colonels and brigadiers and generals were silent. Were they waiting until Mubarak imposed martial law?

The crowds refused to abide by the curfew. In Suez, they set police trucks on fire. Opposite my own hotel, they tried to tip another truck into the Nile. I couldn't get back to Western Cairo over the bridges. The gas grenades were still soaring off the edges into the Nile. But a cop eventually took pity on us – not a quality, I have to say, that was much in evidence yesterday – and led us to the very bank of the Nile. And there was an old Egyptian motorboat, the tourist kind, with plastic flowers and a willing owner. So we sailed back in style, sipping Pepsi. And then a yellow speed boat swept past with two men making victory signs at the crowds on the bridges, a young girl standing in the back, holding a massive banner in her hands. It was the flag of Egypt.

Egypt's day of crisis

President Mubarak's regime called in the army and imposed a curfew after tens of thousands of protesters took to the streets demanding an end to his rule.

Large numbers of protesters defied the curfew in Cairo to storm the state TV building and the Foreign Ministry.

The headquarters of the ruling National Democratic Party were set alight.

Protesters chased riot police away from Cairo's main square. Some police are reported to have removed their uniforms to join the demonstrators.

At least one protester was killed, in Suez, after clashes with police.

Nobel Peace laureate Mohamed El-Baradei, who had returned to Egypt for the protests, was put under house arrest after being hosed by water cannon.

Mobile phone and internet services were disrupted.