It might be the end. It is certainly the beginning of the end. Across Egypt, tens of thousands of Arabs braved tear gas, water cannons, stun grenades and live fire this weekend 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 on Friday even knew where Mubarak – who would later appear on TV to dismiss his cabinet – 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 drug addict ex-policemen, were the front line of the Egyptian state. The true representatives of Hosni Mubarak.

At one point, 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 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. "We have orders. Do you think we want to do this? This country is going downhill." The government imposed a curfew as protesters knelt in prayer before police.

How does one describe a day that may prove to be so giant a page in Egypt's history? Maybe reporters should abandon analyses and just tell what happened in one of the world's most ancient cities. So here it is, the story 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 ElBaradei 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 turned towards the police. "Mubarak, Mubarak," they shouted. "Saudi Arabia is waiting for you." That's when the water cannons 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 ElBaradei, 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 prizewinner who was the top UN nuclear inspector, was drenched like a street urchin. That's what Mubarak thought of him, I suppose: just another troublemaker with a "hidden agenda".

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 his rule was already over. The government had already cut the internet and killed 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 until they drowned out the pop of tear gas grenades. From all over they surged into the city – middle-class youngsters from Gazira, the poor from the slums of Beaulak al-Daqrour, marching 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 into the gutter.

This was Cairo, of course, but these protests were taking place all over Egypt, not least in Suez, where 13 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 battagi arrived, pushing to the front of the police ranks to attack the protesters. They had metal rods and police truncheons and sharpened sticks. They were vicious. 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 Nile's east bank.

The police decided they would hold the east end of the flyover. They opened their ranks again and sent the thugs in to beat the leading protesters. The tear-gassing began in earnest, hundreds of canisters raining on to the crowds. 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 near Mubarak's party HQ. 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, and there, sipping his coffee, was the great Egyptian writer Ibrahim Abdul Meguid. 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 gas. It was one of those scenes that occur in films rather than real life.

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."

Throughout the day we had not seen the army. 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.

I couldn't get back to western Cairo over the bridges, but a cop took pity on us and led us to the 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. A yellow speedboat swept past with two men making victory signs at the crowds on the bridges and a young girl standing in the back, holding a massive banner in her hands. It was the flag of Egypt.