Six people were killed in fighting between government forces and opposition supporters in Yemen's capital Sanaa on Monday, opposition sources said.
The fighting was the first to break out in Sanaa since President Ali Abdullah Saleh flew to Saudi Arabia for treatment after sustaining severe burn wounds when an attempt to assassinate him was made in June.
One clash began when demonstrators seeking to increase pressure on President Saleh to quit marched outside a square where they have been camped for months, the sources said.
Hundreds of youths were confronted by security forces and gunmen in civilian clothes who fired on them, killing one, an opposition figure said.
In northern Sanaa, a family of five was reported to have been killed by shelling during clashes between Republican Guard forces and pro-opposition tribesmen.
In the south of country, a military offensive to retake a provincial capital seized by Islamist militants stretched into a third day. The army said it had killed more than 20 fighters suspected of ties to al Qaeda.
The United States and Saudi Arabia have pressed Mr Saleh to accept a transfer of power plan put forward by Gulf Arab countries.
The president has said he would return to Yemen to lead a dialogue with the opposition and oversee a transition, but critics say this is a stalling tactic.