Egypt's government has ordered police to block access to protest camps set up by supporters of ousted President Mohammed Morsi, state TV informs, BBC reported.
Thousands have been defying warnings from the authorities to abandon the sit-ins in Cairo outside the Rabaa al-Adawiya mosque and at Nahda Square.
On Friday evening they expanded their protest to a third sit-in site.
More than 100 Morsi supporters have been killed in clashes since he was overthrown by the military on 3 July.
The demonstrations are taking place a day after US Secretary of State John Kerry said the generals were in effect restoring democracy when they removed the president.
On Friday, activists said more than 30 protest marches left mosques in Cairo after Friday prayers and made their way to swell the numbers at the sit-in near the Rabaa al-Adawiya mosque, in the east of the capital.
Thousands of Morsi supporters and their families have been camping there for weeks, demanding his reinstatement.
On Wednesday, the government warned protesters to abandon the sit-in, as well the one at Nahda Square near Cairo University in Giza, saying they represented threats to national security.
The Anti-Coup Alliance, a grouping of Morsi supporters led by the Muslim Brotherhood, vowed that the protests would continue.
"All revolutionary groups, including the alliance, also announce that they do not recognise the coup government or its decisions or negotiations," a statement said, adding that it placed "full responsibility on the coup leaders for any acts of violence or killings".
Human Rights Watch also warned the authorities not to clear the sit-ins.
"To avoid another bloodbath, Egypt's civilian rulers need to ensure the ongoing right of protesters to assemble peacefully, and seek alternatives to a forcible dispersal of the crowds," it said.
On Friday afternoon, state TV reported that within the next 48 hours security forces would impose a blockade on the two protest camps, allowing people to leave but not to go in.
The Muslim Brotherhood's Ahrar 25 TV meanwhile said two new sit-ins had begun - one outside the Mustafa Mahmoud mosque in the southern Mohandessin district, and another in Alf Maskan in the east, near the capital's international airport.
The Anti-Coup Alliance said it planned marches to several security installations in Cairo, including the military intelligence and national security headquarters.
Security forces also fired tear gas at Morsi supporters who had gathered outside Media Production City, a complex housing most of Egypt's private television channels.
Later, state-run Nile News TV reported that a policeman had been injured in clashes with protesters there.
The US government has been outspoken in its insistence that the right to peaceful assembly and to stage sit-ins must be respected, reports the BBC's Jim Muir in Cairo.
They have balanced that with comments from the secretary of state, who said Egypt's military had been "restoring democracy" when they overthrow the country's first freely elected president.
"The military was asked to intervene by millions and millions of people," Mr Kerry told Geo TV during a visit to Pakistan. "The military did not take over, to the best of our judgement - so far."
On Friday, Mr Kerry said: "Egypt needs to get back to a new normal. It needs to restore stability, to be able to attract business and put people to work."
"We will work very, very hard together with others, in order to bring parties together to find a peaceful resolution that grows the democracy and respects the rights of everybody."
Mr Kerry's earlier comments were rejected by a leader of the Muslim Brotherhood, the Islamist movement to which Mr Morsi belongs.
"The United States is a country that speaks of democracy and human rights and they say something like that. I hope that they rethink their position and correct it," Mohammed Ali Bishr told the Reuters news agency.
Mr Kerry's deputy, William Burns, arrived in Cairo on Friday night - his second visit to the Egyptian capital since the military took control.
Egyptian foreign ministry spokesman Badr Abelatty said Mr Burns would meet Foreign Minister Nabil Fahmy on Saturday. It is not known whether he will also hold talks with the head of the armed forces, Gen Abdul Fattah al-Sisi, or Mr Morsi, who is being detained by the military.
Vice-President Mohammed ElBaradei told the Washington Post that Gen Sisi "understands that there has to be a political solution", adding: "But of course he has a responsibility to protect the country in terms of security. And the army is on the edge."
BBC correspondent says it will be interesting to see how much traction Mr Burns can muster in his efforts to calm this bitterly polarised situation.