Egyptian protesters have clashed with supporters of President Mohamed Morsi's Muslim Brotherhood, and ransacked three offices nationwide including the group's headquarters in Cairo, leaving at least 90 people injured.

The group's spokesman Ahmed Aref said on Friday that men assaulted women in the office who were holding an event commemorating Mothers' Day, and then forced them into bathrooms before they destroyed the office's contents.

Thousands of activists thronged to the building and battled Brotherhood supporters with birdshot, rocks, knives, sticks and their fists Friday.

Gunshots were also heard ringing in the neighbourhood.

Young men threw stones and wielded tree branches and broken bottles as they chanted against Morsi near the Brotherhood's headquarters in Cairo. One sign held aloft by a protester outside the headquarters read: "Who is ruling Egypt?"

Riot police stood guard around the building but did not interfere to break up the two sides fighting a few blocks away, although they fired tear gas at protesters who approached the headquarters later in the evening.

Black plumes of smoke billowed after protesters torched buses that had ferried Brotherhood members to the site, and security officials said at least 90 people were injured.

Fatima Khalifa, 30, said she was demonstrating to send a message to the Brotherhood that they are the aggressors.

"Morsi must be tried for killings of protesters just like Mubarak,'' she said.

Two Brotherhood offices in the second-largest city of Alexandria and in the Nile Delta city of Mahalla were also attacked.

Anger first erupted last week when Brotherhood members beat journalists and liberal and secular activists during a protest outside the group's Cairo headquarters.

During that protest, journalists claimed they were beaten after being suspected of conspiring with activists by spraying anti-Brotherhood graffiti, allegations that they have denied. 

Deep dissatisfaction 

Muslim Brotherhood spokesman Yasser Mehres blamed opposition parties for calling Friday's protest outside the group's headquarters.

Mehres said it gave way for "thugs" to infiltrate and attack Brotherhood offices.

The Brotherhood spokesman said the group's headquarters was the wrong place to demand change.

"The protesters' demands should be delivered to the government and president, not the Brotherhood office because even though the president came from the group, he makes decisions that are separate from the group,'' Mehres said. 

"Right now, Brotherhood buses are being burnt and there are serious injuries with people in critical condition," he said. "It is not acceptable that Egyptians watch TV and see this farce taking place as Egyptians fight one another.''

Egypt has faced near-constant turmoil in the more than two years since longtime, authoritarian leader Hosni Mubarak was overthrown in a revolt.

Mubarak's successor Morsi, the country's first democratically elected leader, has faced increasing frustration over the slow pace of his efforts to reform the state and fulfil the revolution's promises of better living standards and justice.

Source: Agencies