|At least seven people have been killed in clashes in Cairo and Suez since last week's football violence [EPA]
The US has demanded "clarification" over Cairo's apparent plans to put dozens of pro-democracy activists, including 19 US citizens, on trial over charges of illegal funding of aid groups.
Victoria Nuland, the US state department spokeswoman, said on Monday that Washington was "deeply concerned" over the developments, which threatened to further strain the country's ties with Egypt's military rulers.
"We have seen media reports that judicial officials in Egypt intend to forward a number of cases involving US-funded NGOs to the Cairo criminal court," Nuland said.
"We are deeply concerned by these reports and are seeking clarification from the government of Egypt."
A top official at Freedom House, one of the groups targeted, called Egypt's handling of the matter "a disaster".
A judicial source in Cairo, the capital, told the AFP news agency that 44 people, including Egyptians, would be tried over the alleged illegal funding of aid groups, a day after the US said it would review $1.3bn aid to Egypt over the crackdown.
The offices of Freedom House and the International Republican Institute (IRI) were among 17 local and international NGOs raided in December by Egyptian authorities as part of a probe into funding.
The aid workers are accused of "setting up branches of international organizations in Egypt without a license from the Egyptian government" and of "receiving illegal foreign funding”.
Sam LaHood, the head of the Egypt office of the IRI and the son of US Transportation Secretary Ray LaHood, is among those being held.
A travel ban on all the NGO workers who were detained remains in place.
Meanwhile, protesters demanding swift presidential election and an early handover of power by the ruling military say they will continue their demonstrations after a night of clashes with security forces.
Thousands of people protested outside the interior ministry in Cairo, accusing the ruling Supreme Council of Armed Forces of failing to prevent the deaths of 72 people during last week's football violence.
"The demand is that the army step down politically and announce the start of nominations for the presidential election immediately," Waleed Saleh, an activist, said on Sunday.
Police used tear gas against demonstrators who attempted to storm the interior ministry.
Al Jazeera’s Sherine Tadros, reporting from Cairo, said: "The protesters have set a building and one armed vehicle on fire, while the police have been firing tear gas for several hours. The security forces have also fired bird shot.
"The situation is really deteriorating, looking much like a war zone in that area around the interior ministry building.
"Whereas the fighting was confined to one or two streets in the past few days, now seems to be spreading.
"What is happening is more and more protesters are coming out because they feel their friends and brothers are being injured and killed, and they are going to fight in solidarity," she said.
At least seven people have been killed in clashes since the match in the Mediterranean city of Port Said.
Protesters blame the police for allowing or even encouraging the violence during the match between al-Ahly and al-Masry on February 1.
Meanwhile, the military-backed government’s reported plan to move Hosni Mubarak, the deposed president, to a Cairo prison hospital, appeared to have done little to quell the anger of those demonstrating.
Protesters have long complained the army was sparing its former commander the humiliation of jail by keeping him in a military hospital during his trial.
Mubarak has been charged over the deaths of protesters during the 18-day uprising that deposed him.
Political figures and a civilian advisory body to the military have suggested bringing forward a presidential vote to April or May, from the June date foreseen in the transition timetable of the army, which took power after Mubarak stepped down.
The Muslim Brotherhood, which forms the biggest bloc in a newly elected parliament, added its voice on Saturday to calls for a faster transition.