Supporters of Egypt's deposed president have marched towards security headquarters in Cairo, raising fears of new clashes as the EU's foreign policy chief met local officials.

The marches on Monday night came despite a warning from the National Defence Council late on Sunday that it would take "decisive and firm action" against demonstrators if they went beyond their right to peaceful protest. The military also issued a similar warning to demonstrators not to approach military or security installations.

Tensions have been running high since scores were killed at a rally in support of Mohamed Morsi in Cairo on Saturday.

Khaled al-Khateeb, the head of the central administration of Egypt's emergency services, on Monday revised the toll in the clashes up to 81, not including a police officer who died of his wounds and nine people killed in Alexandria.

Monday night's marches began after backers of Morsi broke their Ramadan fast.

Groups moved from their key sit-in site by the Rabaa al-Adawiya mosque in Cairo towards the interior ministry and two other security forces buildings.

Thousands of demonstrators, men and women chanted slogans against army chief General Abdel Fattah el-Sisi, who led the military's toppling of Morsi on July 3 .

Al Jazeera's D. Parvaz, reporting from Cairo, described the atmosphere at the protest as "very peaceful", and said that demonstrators in one rally had calmly turned around and returned to the square when police barred their way at a distance from the mosque.

Earlier, the Anti-Coup Alliance of groups protesting against Morsi's ouster also called for a massive turnout for demonstrations on Tuesday.

"We... call for a million person march under the banner of 'Martyrs of the Coup' on Tuesday," they said in a statement.

They urged Egyptians "to go out into the streets and squares, to regain their freedom and dignity - that are being usurped by the bloody coup - and for the rights of the martyrs assassinated by its bullets".

The calls raised the possibility of fresh confrontations after the National Defence Council warned protesters "not to exceed their rights to peaceful, responsible expression of their opinions".

The council said demonstrators would face "decisive and firm decisions and actions in response to any violations".

Millions of Egyptians approved of Morsi's overthrow, however, and they turned out on Friday for mass rallies called by Sisi to show their support for the military.

Diplomatic visit

With tensions rising, EU foreign policy chief Catherine Ashton decided to extend by one day a visit that began late on Sunday, meeting with key officials, including the deposed president.

Ashton and Morsi held a two-hour meeting on Monday night, according to a spokesperson for the EU official, but no further details were provided.

Her meeting with Morsi came after similar meetings with interim president Adly Mansour, vice-president for international affairs Mohamed ElBaradei and army chief el-Sisi.

On Monday night, she also met representatives of the pro-Morsi coalition, but was not expected to comment on her discussions until Tuesday.

"There were no initiatives presented by Ashton or us, she just heard updates on the situation since her last visit," Mohamed Ali Bishr, a pro-Morsi delegation member said.

The bloodshed in the Arab world's most populous nation has sparked mounting international concern, and Washington on Monday "strongly" condemned the violence.

"The United States strongly condemns the bloodshed and violence," the White House said in a statement, urging the interim government to respect demonstrators' rights.

A group of Egyptian NGOs issued a statement on Monday calling for Interior Minister Mohamed Ibrahim to be sacked for what it called Saturday's "massacre".

A crackdown on Morsi supporters continued Monday, with the arrest of the president and vice president of the moderate Islamist Wasat party, which has protested against Morsi's removal.

Morsi is being held at an undisclosed location on suspicion of crimes relating to his escape from prison during the 2011 uprising that overthrew president Hosni Mubarak.