The first protesters arrived in the square around 10 a.m., marching from al-Azhar mosque to Tahrir after dawn prayers, al-Masry al-Youm reported. While some were celebrating last year's uprising, others were calling for an end to military control of Egyptian government.
Activists on Tuesday decried the military government's announcement of a partial lifting of emergency law.
Field Marshal Hussein Tantawi, Egypt's de facto head of state, said the military regime would end Egypt's state of emergency -- in effect since 1967, except for an 18-month break in 1980 and 1981 -- on the anniversary of the uprising that ousted President Hosni Mubarak, "except in fighting acts of thuggery."
Tantawi did not elaborate on the term.
"From our perspective, the state of emergency has not been lifted," Egyptian Initiative for Personal Rights Director Hossam Bahgat told the British newspaper The Guardian.
"Tantawi's speech does nothing to deal with the most harmful aspect of the state of emergency, which is allowing the police to retain powers to stop, search and detain anyone they suspect of being a 'thug,' without having to obtain a judicial warrant," he said.
Bahgat said Mubarak himself issued a similar partial repeal a few years ago, vowing to limit the emergency law's application to cases of drug trafficking and terrorism.
But the promise "was routinely violated and only led to the creation of a state of exceptionality that put the police above the law," Bahgat said.
As Tantawi promised to ease martial law, his regime ramped up security measures at government buildings ahead of Wednesday's protests, widely expected to be one of the biggest outbreaks of popular unrest since Mubarak's fall Feb. 11.
Fortifications, including barbed wire, and armed soldiers were added around downtown Cairo's state television building, which was expected to be a protester rallying point, The Guardian said.
High-end hotels boarded up windows. Several airlines canceled flights to Egypt. The U.S. Embassy warned Americans to stay away from the capital's center.
The ruling armed forces council scheduled a series of official celebrations for Wednesday, including military parades, air shows, a specially commissioned operetta and the distribution of prize coupons to citizens on the streets.
But anti-junta demonstrators, organized by 55 revolutionary movements, vowed to ignore the official celebrations and participate in marches and rallies in at least 10 cities, including No. 2 city Alexandria, demanding an immediate end to military rule.
The biggest protests were set for Cairo, with at least 12 major demonstrations merging in the afternoon and converging on Tahrir Square, the focal point of last year's anti-Mubarak demonstrations.
Youth movement activist Ahmed Bahgat told al-Ahram daily he saw a contrast between last year's demonstrations and Wednesday's.
"Even though everyone wants an end to military rule, there's no consensus as to what the immediate solution should be," he said.
Some activists want an immediate handover of power to Egypt's new Parliament, while others demand the formation of a "national salvation" government, al-Ahram said. Still others want earlier-than-scheduled presidential elections, the newspaper said.