Tens of thousands of Egyptian Islamists gathered in a show of strength Friday ahead of planned opposition protests against President Mohamed Morsi, highlighting tense political divisions in the Arab world's most populous state.

Carrying Egyptian flags and portraits of the president, they packed the large square outside Cairo's Rabaa al-Adawiya mosque in the Nasr City neighbourhood and surrounding avenues.

Islamist groups led by the powerful Muslim Brotherhood, from which Morsi hails, had called the rally ahead of planned June 30 protests to demand an early presidential election.

Morsi has been in office for just one year.

Inside the mosque, after prayers worshippers broke into chants of "Morsi is a president for all Egyptians" before joining the crowds outside.

They called the turnout proof that Morsi enjoyed the support of the people.

"We are here in such huge numbers so that the secularists don't think we are a minority... We are capable of protecting legitimacy and sharia (Islamic law)," said Hamida Bakkout, 43.

Many Morsi supporters had been bused to Cairo for the event, AFP reporters said.

Omar Mostafa, 18, who had come from the Nile Delta province of Beheira, said: "This is a message that there are many of us behind the president. We don't care about the mobilisation of the opposition."

Several media crews covering the protest were forced to leave after Morsi supporters pelted them with bottles of water and damaged some of their equipment.

They blamed the media for "trying to bring down the Islamist project", BBC reporter Mahmud Abu Bakr, one of those attacked, told AFP.

Several blocks away from the rally, hundreds of anti-Morsi supporters gathered near the defence ministry, calling on the army to take power.

In the Mediterranean city of Alexandria, participants in an anti-Morsi march traded insults with his supporters, leading to brief scuffles outside the Qaet Ibrahim mosque, state media reported.

The Islamists accuse the opposition of being remnants of the regime of ousted president Hosni Mubarak and of seeking to sow chaos.

"Democratically elected presidents are never removed through protests," Muslim Brotherhood spokesman Gehad al-Haddad said.

A campaign dubbed Tamarod (rebellion in Arabic) called the June 30 rally to coincide with the first anniversary of Morsi becoming president.

He was elected after a military-led transition that followed Mubarak's ouster in a 2011 popular uprising.

As a senior leader of the Brotherhood, banned but tolerated under Mubarak, Morsi vowed to be a president "to all Egyptians".

But since taking office, he has squared off with the judiciary, media, police and most recently artists, and his opponents accuse him of giving the Islamists a monopoly over public institutions.

Tamarod organisers said they have collected 15 million signatures demanding that Morsi quit, leaving the government jittery and energising the fragmented opposition.

Morsi supporters insist he is cleansing institutions of decades of corruption, and have condemned the June 30 protests as a "coup against democracy".

With bitter political divisions repeatedly spilling onto the streets in violent and sometimes deadly clashes over the past year, the Islamists have accused the opposition of seeking to sow chaos.

A spokesman for the Brotherhood's Freedom and Justice Party, Ahmed Aqil, urged protesters "to embrace peaceful expression of opinion".

"We seek stability in order to rebuild the nation. Violent demonstrations cannot establish a stable regime. Those who say 'President Morsi will be toppled on June 30' live in an illusion they must give up," he said on the FJP website.

US Ambassador Anne Patterson urged protesters to organise rather than take to the streets, provoking fury in opposition quarters.

"Some say that street action will produce better results than elections. To be honest, my government and I are deeply sceptical," Patterson said in a speech this week.

"We oppose chaos. Chaos is a breeding ground for instability... I recommend Egyptians get organised. Join or start a political party that reflects your values and aspirations."

In the latest move to roil the opposition, Morsi appointed 17 new provincial governors on Sunday, including seven from the Brotherhood.

He also named as governor of Luxor a member of an Islamist group whose militants massacred 58 foreign tourists there in 1997, prompting the tourism minister to resign in protest.

The appointments led to clashes in several Nile Delta provinces in what some fear is a prelude to more serious confrontations at the end of the month.

Islamist groups have yet to announce whether they will take to the streets on June 30.