Yemeni government forces have clashed with the country's most powerful tribal group as thousands of the president's supporters and opponents held rival demonstrations.

The fighting erupted on Friday in the capital, Sana'a, where security forces traded gunfire with members of the Hashid confederation. The tribal alliance announced in March that it would back President Ali Abdullah Saleh's opponents. There was no immediate word on what sparked the unrest.

Meanwhile, the president's supporters and anti-government protesters took to the streets after midday prayers for separate rallies on Friday. Activists staged large demonstrations in the capital and the southern city of Taiz.

Mr. Saleh has been clinging to power, even though he has been out of the country since June. He remains in Saudi Arabia where he has been recovering from injuries sustained in a June attack on his presidential compound.

Government opponents have been demanding his resignation. The opposition has rejected Mr. Saleh's calls to negotiate, saying talks can only take place after he agrees to leave office.