Leaders of the different Yemeni tribes gathered in the capital Sana'a on Tuesday in a bid to mediate between a group of armed tribesmen and security forces who have been fighting for two days, dpa reported.
Security forces and followers of influential tribe leader Sheikh Sadiq al-Ahmar have clashed, with the sides firing at each other with machine guns.
The fighting took place outside the house of al-Ahmar, the leader of the Hashid tribe - to which President Ali Abdullah Saleh belongs. Al-Ahmar has expressed his support for anti-government protesters calling for Saleh to step down after 32 years in power.
Senior tribesman Hassan al-Agdaa, who is taking part in the mediation process, blamed security officials for the renewal of clashes after they used artillery and missiles against al-Ahmar's followers.
He warned that these "clashes might be the beginning of the civil war that the president threatened will erupt."
Saleh has repeatedly accused the country's opposition of dragging the country on the brink of civil war.
At least six tribesmen were killed and almost 70 injured in the two-day clashes, al-Agdaa said.
"Some tribal leaders called President Saleh and he promised them that the shooting will stop. But, we were surprised to find security firing again half an hour later," al-Agdaa told the German Press Agency dpa by phone.
Mediators remain with Sheikh al-Ahmar in his house, with thousands heading there vowing to support him.
The violence comes after the embattled president refused to sign a Gulf-brokered power transition deal, which would end his 32-year-old rule.
Government buildings around the area were closed, witnesses said.
Yemen's state television showed footage of security members and civilians injured in hospitals. One civilians said al-Ahmar's followers were firing at people from the buildings surrounding the house.
The opposition coalition Joint Meeting Parties (JMP) condemned the criminal act of security members who attempted to enter al-Ahmar's house by force.
"We call on Yemeni people to thwart Saleh's plans to drag the country to civil war and urge them to join the peaceful revolution," the JMP said.
Youth activists and tribes leaders in the south-western Aden and the south-eastern Hadramaut provinces also rejected the attack "as an unforgivable crime."
At least 140 people have been killed and thousands injured since the uprising in Yemen started earlier this year.