By Tom Perry CAIRO (Reuters) - Egyptian President Mohamed Mursi threatened on Sunday to take unspecified steps to "protect this nation" after violent demonstrations against his Muslim Brotherhood, using vague but severe language that the opposition said heralded a crackdown. In remarks following clashes outside the Brotherhood's Cairo headquarters on Friday, Mursi warned that "necessary measures" would be taken against any politicians shown to be involved in what he described as violence and rioting.

Two years after toppling Hosni Mubarak, some Egyptian activists say they are having to resort to violence against the ruling Muslim Brotherhood, which they say mirrors the old regime's tactics on dissent. "It's turned into a war on the streets. With every strike against us, we strike back harder," said Farid Sayyed, 24, during clashes between Islamists and opponents outside the Cairo headquarters of the Muslim Brotherhood.

Police were out in force on Saturday outside the headquarters of Egypt's Muslim Brotherhood a day after more than 160 people were hurt in running street battles between Islamists and opposition protesters. An AFP journalist reported a heavy police presence at the building in the Mokattam neighbourhood, and the official MENA news agency cited a health ministry spokesman as saying more than 160 people were injured the night before. Businesses in the area remained closed, and debris from the violence was visible everywhere.

Opposition protesters clashed with Islamists near Muslim Brotherhood headquarters in Cairo on Friday after activists marched to the building guarded by police and members of Egypt's ruling movement. Gunshots were heard as hundreds of opposition activists and the Islamists battled in the streets of the Mokattam neighbourhood where the headquarters are situated, but there were no immediate reports of casualties. Police had fired tear gas at the protesters outside the headquarters before the clashes moved elsewhere in the neighbourhood.