Egyptian riot police remove protesters after clashes at Tahrir Square in Cairo November 19, 2011. [Photo/Agencies]

CAIRO - Police and protesters clashed for control of downtown Cairo's Tahrir Square on Saturday, after security forces tried to stop activists from staging a long-term sit-in in the symbolic site.

Protesters attacked a police armored truck, shaking it and pummeling it with rocks. Earlier in the day, riot police beat protesters and dismantled a small tent city set up to commemorate revolutionary martyrs.

The clashes occurred after activists camped in the central square overnight following a massive Friday rally. The military tolerates daytime demonstrations in the central square, a symbol of the country's Jan 25-Feb 11 uprising, but claims that long-term occupation paralyzes the city.

Friday's rally was dominated by Islamists, but the sit-in appeared to be staged mostly by members of left- and liberal-leaning revolutionary youth groups.

The number of protesters swelled to nearly 600 people as news of the scuffles spread in the city, and thousands more riot police streamed into Tahrir Square blocking off the entrances and clashing with protesters.

Police were seen beating activists who challenged them and an Associated Press cameraman saw police arrest three people who refused to leave.

Stubborn protesters played cat-and-mouse with riot police as they were chased outside of the square and into side streets. "We are using side streets to pretend to run errands, but we are just regrouping and going back," said Sahar Abdel-Mohsen, who joined in the protest on Saturday afternoon, after a call went out on Twitter telling people to come down to Tahrir.

Protesters were chanting anti-security slogans including, "Riot Police are Thugs and Thieves" and "Down with the Marshal" referring to Field Marshal Hussein Tantawi, Egypt's military ruler.

On Friday, tens of thousands of Islamists as well as leftists and other young activists had massed in Tahrir Square, confronting Egypt's ruling military council with the largest crowd in months to protest a document which would give the generals special powers over a future elected government.

Most of this year's rallies in Tahrir Square since Mubarak's ouster have been led by liberal- or left-leaning groups, but Islamists dominated Friday's protest.

While united against giving the army new powers, however, Islamists and liberals were jockeying among themselves for votes in crucial parliamentary elections only 10 days away.

The stakes are higher for all sides than at any time since the uprising ousted President Hosni Mubarak in February. The victors will help choose who will draft a new constitution, thus defining the character of post-revolutionary Egypt.