Egypt's military appealed for conciliation and warned against revenge attacks, after it toppled president Mohammed Morsi, as police rounded up senior Islamists ahead of planned rallies by Morsi's supporters on Friday.

The military published the statement on its spokesman's Facebook page as scores were injured in clashes between the Islamist Morsi's supporters and opponents in the Nile Delta ahead of the planned rallies.

The United States on Thursday pressed Egyptian officials to avoid the "arbitrary arrests" of ousted Islamist president Mohamed Morsi and his supporters, an administration official said.

Members of President Barack Obama's national security team have also stressed the importance of a "quick and responsible return" to elected civilian government in Cairo, in contacts with Egyptian officials and Washington's regional partners, the U.S. official said, on condition of anonymity.

U.N. leader Ban Ki-moon on Thursday told Egypt's foreign minister that there must be a "quick return" to civilian rule in the country after the military ousted president Mohammed Morsi, a spokeswoman said.

Ban also renewed international criticism of the military action against Egypt's first freely elected leader when he held talks with Foreign Minister Mohamed Kamel Amr, said U.N. spokeswoman Eri Kaneko.

Anger gave way to gloom on Thursday as Islamists rallied at a Cairo mosque where thousands of supporters of deposed president Mohammed Morsi said they felt under siege and betrayed by the army.

Pictures of the ousted leader hung on walls, as protesters chanted against the military that removed Morsi from his post and outlined a roadmap for a political transition.

The African Union will meet Friday to discuss the crisis in Egypt, with delegates seen likely to suspend Cairo in line with strict rules against unconstitutional changes of government, officials said.

The pan-African bloc's Peace and Security Council will meet Friday morning at AU headquarters in the Ethiopian capital to discuss the ousting of president Mohammed Morsi, the organisation said on Thursday.

Jordan's Muslim Brotherhood on Thursday condemned the military coup that has ousted Egypt's Islamist president Mohamed Morsi, calling it a "U.S.-led conspiracy."

"We condemn the military coup against Egypt's first democratically elected and legitimate president," the Brotherhood's shura advisory council said in a statement on its website after what is said was an emergency meeting.

An alliance of Islamist parties and movements including ousted president Mohammed Morsi's Muslim Brotherhood has called for "peaceful protests" on Friday to denounce the military's overthrow of the Islamist president.

The National Alliance to Support Legitimacy called for "peaceful protests on Friday in all of Egypt's provinces to denounce the military coup against legitimacy and in support of the legitimacy of President Morsi".

Egyptian military police on Thursday arrested Muslim Brotherhood supreme guide Mohammed Badie, a security official said, as authorities rounded up members of the influential group from which ousted president Mohammed Morsi hails.

Badie "was arrested in (the western city of) Marsa Matrouh at the request of the prosecution for inciting the killing of protesters," the official told Agence France Presse.

NATO Secretary General Anders Fogh Rasmussen on Thursday called for a quick return to civilian government in Egypt.

"I am gravely concerned by the situation in Egypt and ... reports of fighting and deaths" given the country's "vital role" in the region, Rasmussen told a news conference.

Qatar, a key political and financial backer of Egypt's deposed Islamist president Mohammed Morsi, congratulated his successor on Thursday while pro-government newspapers warned of possible future violence in the country.

"Qatar will continue to back Egypt in its leading role in the Arab and Muslim worlds," a foreign ministry spokesman said in remarks carried by the official QNA news agency.