Tens of thousands of Islamists poured onto Egypt's streets on Friday demanding the reinstatement of ousted president Mohammed Morsi, despite warnings by the military of a crackdown on violent protests.

The rallies come a day after Morsi's army-installed successor Adly Mansour vowed to fight for stability against opponents he accused of wanting to plunge the crisis-hit country "into the unknown".

The U.N. is seeking answers from Egypt over the arrests of former president Mohammed Morsi and others following the overthrow of his government on July 3, a spokesman said Friday.

The spokesman for the U.N.'s High Commissioner for human rights, Navi Pillay, said she had called in Egypt's ambassador in Geneva on July 10 and also written to authorities in Cairo for an explanation of events earlier this month.

Britain on Friday announced it was revoking export licences for equipment used by Egypt's military and police amid concerns it could be used against protesters.

The licences concerned cover components for armored personnel carriers and for machine guns, as well as radio and communications equipment, including for tanks, a spokeswoman for the business ministry told Agence France Presse.

Egyptian President Adly Mansour vowed on Thursday to battle for security "to the end", as Islamists promised new protests against his army-installed interim government.

"We are at a decisive moment in Egypt's history, which some want to steer into the unknown," he said in a televised address.

Egypt's Muslim Brotherhood would accept EU mediation in talks to restore ousted president Mohammed Morsi, even briefly before new elections, but is still planning sustained protests, officials said Thursday.

Brotherhood officials who met EU foreign policy chief Catherine Ashton and envoy Bernardino Leon said they believe European Union mediation could eventually lay the groundwork for a roadmap to restore Morsi's presidency before an election.

Ten jihadists have been killed in Egypt's restive Sinai peninsula in the past two days during an army offensive against Islamist militants, the official MENA news agency reported on Thursday.

"The security operation carried out by the armed forces in northern Sinai in the past 48 hours led to the deaths of 10 jihadists," a military source was quoted as saying.

Turkish Prime Minister Recep Tayyip Erdogan has refused to speak to Egypt's new Vice-President Mohammed ElBaradei, the latest broadside in a spat that erupted after the military coup in the Arab world's most populous country.

Erdogan infuriated Egypt's interim leaders after he voiced support for ousted Islamist president Mohammed Morsi.

Egypt's Muslim Brotherhood slammed the EU's failure to denounce the military coup that ousted president Mohammed Morsi, after the bloc's foreign policy chief met Islamist officials and called for his release.

Catherine Ashton met senior pro-Morsi figures during her visit to Cairo on Wednesday, including former premier Hisham Qandil and representatives from the Freedom and Justice Party, the Brotherhood's political wing.

Gunmen shot dead a policeman on Wednesday in El-Arish, in Egypt's restive Sinai peninsula, medical sources said.

The militants opened fire on the police conscript who was standing outside a police station in north Sinai's main town, hitting him in the neck before fleeing in their vehicle.

EU foreign policy chief Catherine Ashton said Wednesday that she regretted not having been able to meet Egypt's ousted president Mohammed Morsi on a visit to Cairo.

She called for his immediate release from the custody he has been kept in since just hours after the military toppled him on July 3 following nationwide street protests against him.