Government forces in Yemen have reportedly carried out air strikes against tribal forces opposed to President Ali Abdullah Saleh.
Air force planes struck an area in Naham province, north-east of the capital, Sanaa, leaving an unknown number of people dead and injured.
Government and tribal forces have been battling each other for days, over the president's refusal to hand over power.
Supporters of the government and the opposition have both called for large demonstrations later.
The BBC reports the air strikes appear to mark a further escalation in recent fighting.
Thousands of people gathered after Friday prayers in Sanaa, carrying the coffins of about 30 people killed in clashes this week. White doves were released.
President Saleh has faced a wave of popular protests similar to those seen in other Arab countries, as well as a mounting challenge from a rival clan.
He has repeatedly refused to sign a deal that would see him transfer power within a month to a unity government.
More than 80 people have been killed since Monday in fighting between the Hashid tribal federation, which is led by Sheikh Sadiq al-Ahmar, and security forces.