CAIRO — Pockets of rebellion echoed across Cairo on Wednesday as security forces tightened their grip on the capital and activists tried not to lose the momentum sparked a day earlier by an unprecedented nationwide protest against President Hosni Mubarak.

The Interior Ministry — stunned by the size and passion of Tuesday’s demonstrations — announced it would not tolerate further protests. Activists in parts of the city defied the ministry’s threats of “”immediate arrest.”” But the crackdown appeared to keep thousands of protesters, angered by unemployment and repression, from venturing back into the streets.

The April 6th Youth Movement, which organizes protests through Facebook, said it was not deterred by a police presence that grew larger throughout the day. The group, which wants to topple Mubarak’s three-decade-old government, said it was planning a large demonstration after Friday prayers, a provocation that would likely trigger unrest not seen since Egypt’s deadly “”bread riots”” of 1977.

The day was marked by police quickly chasing protesters away as they attempted to gather. More than 2,000 demonstrators arrived at a courthouse near the National Museum. Minutes later, police closed in, scattering the dissidents, some of whom threw rocks and set tires on fire as they fled. Protesters were often out of communication with one another, as Twitter and other social networking accounts were blocked.

Authorities said a man in the city of Suez became the fourth person, including a policeman, to die of injuries in protests. A witness says a government building in Suez was also on fire Wednesday night.

At least 500 people have reportedly been arrested this week, scores of them before dawn on Wednesday, when police using water cannons and tear gas dispersed a crowd of several thousand hunkered in Cairo’s Tahrir Square.

“”The harshness and brutality of the police has scared a lot of people,”” Fathi Abdul Latif, a member of the opposition National Front for Change, said as police swung bamboo canes and hauled off five protesters near the Journalists’ Union. “”Activists and organizers are regrouping. A revolution needs time. What happened on Tuesday has given us confidence.””

As he spoke, though, a crowd of about 70 demonstrators, far outnumbered by police and passers-by, who flashed pictures with cell phone cameras, chanted: “”One, two, where are the Egyptian people?””

That question perturbed many activists. They wondered how, a day after more than 10,000 people turned out in one the capital’s largest demonstrations in decades, Cairo could return to the workday rhythms of lunch breaks and traffic. Those scenes, however, were occasionally interrupted by updates on social networking sites announcing spasms of revolt, including police firing tear gas to break up protesters assembling in several neighborhoods.

“”We have to get everybody out into the streets,”” said Ali Ebeid, a medical worker who threw his fist into the air in front of a line of riot police. “”If we don’t, we could lose the moment.””

That moment was inspired by the Tunisia uprising that toppled the autocratic regime of President Zine El Abidine Ben Ali. Like their North Africa counterparts, many Egyptians are angry over three decades of a government that offers little hope to the young, who blame the ruling party for corruption, unemployment and stagnation. It is the effects of these failings in ordinary lives, not ideology or the urgings of political opposition groups, that Egyptians say are driving their resentment against a president many regard as a dictator.

The 82-year-old Mubarak, who may seek re-election this year, has watched his popularity steadily tumble as Egyptians, who rarely ridiculed him in the past, openly yell epithets against him. It is a turn of fate many find hard to comprehend, despite years of mass arrests, especially against the Muslim Brotherhood, and the silencing of many political opponents.

“”I’ve been a political analyst for 30 years and I didn’t expect this,”” said Diaa Rashwan. “”This has opened a new political history in Egypt. It’s the first time people are deciding for themselves to protest and demand. Everybody had expected the lower classes to one day revolt, but these protesters are the educated, the middle class and even women.””

As dusk fell, Tahrir Square filled with policemen who were clearing sidewalks, hurrying pedestrians along and waiting at the mouths of streets for hints of unrest.

“”In a country like Egypt as in most authoritarian regimes, the power of security forces eventually collapses,”” said Nabil Abdel Fattah, an analyst with Al-Ahram Center for Political and Strategic Studies. “”They’re not used to dealing with these kinds of protests, and if they go on for days, they will break the hold of police. It’s a major turning point.””

On Wednesday, a protest is exactly what Mohamed Adawi, an accountant, was looking for as he combed Cairo streets looking for a crowd and listening for the sounds of marching feet.

“”I’m really frustrated that today I didn’t have a chance to join a protest,”” he said. “”We made a great achievement on Tuesday, but we will lose that if we can’t sustain the same protests over the next few days. We might even go back to the days when carrying out a demonstration was impossible.””