If the Egyptian authorities were taken aback by the ferocity of this week's protests, yesterday they struck back as protesters were arrested across the city.

At Tahrir Square, the centre of most of Tuesday's clashes, there was a tense calm. Riot police, some carrying guns, stood beside lines of riot trucks lined up along roads in the plaza, which borders the world-famous Egyptian museum.

Police used tear-gas to disperse hundreds of protesters in nearby Ramses Street, while lines of riot squad officers surrounded other protesters in central Cairo.

I went to Cairo University, in the west of the city, where activists had earlier used Twitter to call for a march. The response came in the form of more than 20 riot trucks. As a group of students made calls to see where the day's marches were taking place, six plain-clothed policemen approached. We were grabbed and herded into a nearby white van.

One of the policeman started screaming, demanding that all phones and identification were handed over. An activist in the front of the van was slapped repeatedly around the face by a burly officer sitting behind him, while another policeman barked at the demonstrators to give him their names, addresses and occupations.

After about 15 minutes, three of us were released, but five young men were driven away. Noha Tarek, a 25-year-old international relations student, was one of those seized by the officers. She said: "They are trying to scare us. They want to take our information so that we know they are watching us.

"They don't want us to go out onto the streets," she added. "But I will still go. They have my name, my job and my age, so now I have nothing to be afraid of."