Thousands of people rallied in Cairo's Tahrir Square on Friday calling for national unity after attacks on Egyptian churches, and solidarity with the Palestinians.

Some held up crosses and others waved Palestinian flags as the numbers swelled in Cairo's iconic square, the epicentre of protests that overthrew president Hosni Mubarak in February.

"If you attack a Christian, you're attacking all Egyptians," said one man giving a speech at a podium.

"The churches attacked in Imbaba are not less than the mosques attacked in Jerusalem," he said, linking the two themes of Friday's protest.

"National unity was there during the revolt but the remnants of the old regime want to destroy the country," said Ahmed Muhanna wearing a green headband bearing the words "the army of Mohammed".

Fifteen people were killed in clashes last Saturday after Muslims surrounded a church in Cairo demanding the handover of a woman they said Christians had detained after she converted to Islam and married a Muslim.

The Muslims also set fire to a second church.

The unrest threatened to drive Egypt's often tense religious tensions to the brink, prompting the military to arrest more than 200 people it said will swiftly be tried.

Activists had called for a mass show of unity on Friday, which has become a regular day of protest after the weekly Muslim prayers at noon.

The demonstrators who gathered in the square ahead of the prayers also waved Palestinian flags.

Activists have called for a march to neighbouring Gaza at the weekend to show solidarity with the Palestinians as they mark the "Nakba" or "catastrophe" which befell them following Israel's establishment in 1948.

The interior ministry has urged them to cancel the march.

The young Egyptian woman at the centre of last weekend's clashes was arrested on Thursday and charged with marrying more than one husband, a judicial source said.

Abeer Talaat Fakhry, 26, was living with her Christian husband in the southern city of Assiut when she ran away from home, converted to Islam and informally married Muslim Yassin Thabet.

Copts account for up to 10 per cent of the country's 80 million people. They complain of discrimination and have been the target of repeated sectarian attack.

The most recent violence has been blamed on a hardline Islamist sect, the Salafists, who have regularly staged protests demanding the church release women they believe converted to Islam.

The sect was mostly apolitical under Mr Mubarak, but since February it has grown more assertive and its leaders say they will form parties to contest a parliamentary election in September.

- AFP