At least 14 people have been injured when unknown assailants attacked opponents of Egyptian President Mohammed Morsi who were camping outside the presidential palace in Cairo, health officials said.

Around 30 unmasked attackers on Saturday threw petrol bombs at the protesters' tents and fired pellets in the air outside the palace in the eastern Cairo quarter of Heliopolis, said witnesses.

The injured included seven police officers who were on duty near the palace, the Interior Ministry said.

The state-run newspaper al-Ahram, citing a security official, said police were intensifying efforts to identify and arrest the assailants.

Morsi had left the palace before the attack, added al-Ahram, quoting a presidential source.

The site was the scene of deadly violence between the president's backers and opponents in early December.

The protesters have camped out near the palace for more than a month now, decrying a constitution drafted by a Muslim Brotherhood-dominated constituent assembly and Morsi's insistence to put it up for a referendum.

The charter was approved late last month by nearly 64 per cent of those who cast their ballots, although only one third of the registered votes showed up.

Saturday's violence comes almost two weeks before mass rallies called by the opposition to mark the second anniversary of the eruption of an uprising that eventually deposed former President Hosni Mubarak.

At least 10 people died in clashes outside the palace on 5 December that broke out when supporters of Morsi attacked the sit-in. Some were reportedly killed by gunfire.

On 31 December, gunmen shot and critically wounded a well-known activist at the site of another sit-in in downtown Cairo's Tahrir square.