|Egypt's best organised opposition group Muslim Brotherhood has stayed away from the protests [AFP]
Thousands of protesters have returned to downtown Cairo's Tahrir Square for what they called a "second revolution", calling for Egypt's military rulers to speed up the pace of democratic reforms in a country that is still charting its political future.
Protesters streamed into Tahrir Square - the symbolic heart of protests that toppled Hosni Mubarak on February 11 - carrying banners reading "Egyptian revolution is not over".
Youth groups that had helped to launch the uprising against Mubarak have dubbed Friday "the second day of anger".
In a Facebook call, the Coalition of the Revolution Youth, which consists of several movements behind the Egyptian uprising, urged protesters to rally for "an end to political corruption."
A large banner hung over the flag-waving crowd, demanding a new constitution "now and not later".
But Muslim Brotherhood, the country's best organised opposition movement, said on Thursday it was "very concerned" by Friday's protest.
The group, that decided not to participate in the demonstrations, said the revolution had achieved many of its goals, including not only the ouster of Mubarak, but also his referral to trial along with his sons and associates.
The call to protest can therefore "only mean that the anger is directed at the people themselves or at the army," said the group, urging protesters not to divide the people and the military.
Al Jazeera's Ayman Mohyeldin, reporting from Cairo, said that the Muslim Brotherhood had given several reasons why they did not believe this was an appropriate time for them to participate, even though they did support the right of public protests and peaceful demonstration.
"The Brotherhood disagrees with the protesters' demands that the constitution be rewritten prior to the elections," he said.
"While some of the protesters are calling for a civilian council to be sitting alongside the military council currently ruling the country, the Brotherhood disagrees with this."
"They say there is already a national referendum that was held several weeks ago in which an overwhelming majority of people agreed to having a constitution amended by an incoming parliament that would be elected in September. They don't want to see that overruled by the military leadership of this country."
Military to stay away
Also, the military said in a statement on Thursday that it would steer clear of protests in an effort to avert any unrest.
It warned in a statement on its Facebook page of "suspicious elements who will try to pit the military against the people," and said it "decided to have completely no presence in areas of protests to avert these dangers".
Mohyeldin said there there was a bit of anxiety at the military's absence at today's protests at Tahrir Square.
"It has triggered an alarm bells with some others protesters, as military was giving green light for people to potentially disrupt the peaceful protests. There were others who felt it was a good step on the military to avoid any friction between the military and protesters as there has been some tension as of late," Mohyeldin said.
Mohamed ElBaradei, the Egyptian Nobel Peace Prize winner and a reform leader, said that he was "seriously concerned about the absence of security forces".
The activists have called for the nationwide protests against what they see as foot-dragging on reforms and trials of former officials for graft and abuse of power.
Egyptians fearing social chaos accepted the army's role in keeping order after Mubarak's overthrow, but many now bemoan a worsening security situation and delays in bringing former officials accused of corruption to trial.
Activists say the military has tried hundreds of civilians in military courts and hindered peaceful protests by arresting activists and holding them without charge.
Authorities detained four activists on Thursday for putting up posters calling for the protests. The four, including film director Aida al-Kashef, were held by military police in Cairo.
A military source said they were released after several hours in detention.
Protesters want the acceleration of trials of former regime figures and their removal from top jobs in police, universities and other public institutions.
They are also calling for a return of security forces to the streets, amid weeks of insecurity and sectarian clashes blamed on remnants of the old regime.