Egypt declared a state of high alert this morning as protesters stormed the Israeli embassy in Cairo and Israeli diplomatic staff left the country.

The crowd pulled down the Israeli flag and tossed documents out of windows in the most serious sign of deteriorating relations since the removal of Egyptian President Hosni Mubarak.

Hundreds of Egyptian soldiers backed by armoured cars were rushed to the embassy after US President President Barack Obama called on Egypt to protect the Israeli embassy. They were, however, too late to stop the embassy from being ransacked.

The protesters torched police trucks and attacked regional police headquarters nearby.

Egypt's Interior Minister Mansur al-Eissawy declared a state of high alert and the government announced it was convening an emergency meeting to deal with the crisis.

Israel's ambassador flew out of the country, heading back to Israel.

Israeli Defence Minister Ehud Barak called US Defence Secretary Leon Panetta early this morning to request help protecting their embassy in Cairo, a statement from his office said.

Hours after the violence broke out, Egyptian soldiers backed by armoured personel carriers massed near the embassy as power was cut to street lights in several blocks around the mission.

Protesters played cat-and-mouse with police throughout the night, amid clouds of tear gas and smoke from burning tyres.

Thousands outside the embassy had jostled to grab the documents fluttering down from one of the top floors of the highrise where the embassy is located.

The documents, in Arabic, English and Hebrew, bore the watermarks of the embassy. They ranged from requests to Egyptian authorities for weapons permits for embassy security to internal correspondence on vacations.

State television quoted an interior ministry official as saying that "foreign hands" were behind the violence. Egypt's rulers often blame foreigners for unrest in the country.

Earlier, thousands of protesters had massed in Tahrir Square to demand reforms and an end to military trials of civilians.

After listening to the weekly Muslim prayer, which told Egyptians it was shameful to "forget their revolution," about 1,000 people left the square and marched to the Israeli embassy several kilometres (miles) away.

Chanting "Lift your head high, you are an Egyptian," they demolished the security wall outside the mission with sledge-hammers and a hefty metal bar, as military police looked on.

The wall, about two metres (6.5 feet) high, consists of prefabricated cement slabs recently installed around the building that houses the embassy overlooking a bridge in Cairo.

Motorists on the bridge adjacent honked horns in support as some protesters chanted: "To Jerusalem we will march, one million martyrs!"

One protester clambered up the embassy building and removed the flag, throwing it down to the rapturous crowd below.

Protesters set fire to two police trucks around the embassy building, and pelted anti-riot police with stones, an AFP journalist witnessed.

They grabbed several helmets and shields from police and at least one teargas gun, while others invaded and damaged a small police station in the neighbourhood. Gunfire was later heard in the area.

Relations between Egypt -- the first Arab country to establish diplomatic relations with Israel in 1979 -- and Israel have been particularly tense since August 18, when Israeli troops killed five Egyptian policemen as they chased militants along the border.

That incident followed a series of Negev desert ambushes that killed eight Israelis.

At the time, outraged Egyptians staged huge protests outside the embassy and called for the expulsion of the Israeli ambassador. Egypt has asked Israel for an official apology and demanded a probe into the deaths.

Since president Hosni Mubarak's ouster in February after a popular revolt, activists have called for a revision of the peace treaty. Mubarak was seen as one of Israel's closest regional allies.

The embassy protest sprang out of a pro-reform rally yesterday in central Cairo, in which protesters chanted slogans against the military ruler and current de facto head of state, Field Marshal Hussein Tantawi.

Protesters also gathered outside the interior ministry to protest earlier clashes with police in which nearly 80 people were injured and dozens of cars torched on Tuesday night.

The protest was called by mostly secular and leftist activists, and was boycotted by the influential Muslim Brotherhood movement and other Islamist groups.