A Yemeni activist says armed government opponents have clashed with police in the southern province of Aden, leaving three Republican Guard troops wounded.

Adnan Abdel-Mohsen says a group of unknown armed people attacked a checkpoint with a bomb in the provincial capital, Krater, 186 miles (300 kilometers) south of the Yemeni capital.

He says the checkpoint was near the Aden branch of Yemen's central bank and on the road to the presidential palace.

Abdel-Mohsen says the police began to shoot indiscrimnately and gun battles have broken out in Krater's streets.

Thursday's clashes took place during more than two months of street protests demanding President Ali Abdullah Saleh step down after 32 years in power.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

SANAA, Yemen (AP) _ Tens of thousands rallied across Yemen on Thursday, denouncing the killing of 12 protesters the previous day in Sanaa and demanding the country's longtime ruler immediately step down.

Demonstrators defied heavy rains and took to the streets to condemn Wednesday's brutal crackdown by forces loyal to President Ali Abdullah Saleh in Sanaa and several other Yemeni cities.

Wednesday's violence drew a condemnation from the U.S. Embassy, which said in a statement it was distressed by the killings. It urged Yemenis to avoid "all provocative demonstrations, marches, and speeches in the coming days" and called on security forces to refrain from using violence against demonstrators.

Saleh has clung to office despite near-daily protests calling for his ouster since early February in a popular uprising inspired by revolts in Tunisia and Egypt.

On Wednesday, security forces backed by snipers on rooftops, fired into about 100,000 people demonstrating in Sanaa, killing 12 and wounding nearly 200.

Opposition parties said in a joint statement that Wednesday's killings were a "savage massacre" and constituted a crime against humanity by Saleh and members of his family who head special branches of the military that fired on the protesters.

Meanwhile, foreign ministers of the six-nation Gulf Cooperation Council will meet Sunday in the Saudi capital Riyadh to fine-tune a draft proposal, accepted by Saleh and the opposition, for ending Yemen's crisis. The proposal calls for the creation of national unity government and would have Saleh transfer power to his vice president within 30 days of the signing of the deal.

In exchange, Saleh and his family would receive immunity from prosecution.

The street protesters have rejected the GCC plan, insisting that Saleh must immediately step down.

The opposition parties, however, warned that they might not sign the agreement if the killings continued.

"Without securing the protection of peaceful protesters, we will not find ourselves able to sign the agreement," they said in the statement, warning that killing protesters would not help Saleh remain in power.

Maj. Gen. Ali Mohsen al-Ahmar, commander of the 1st Armored Division who joined the uprising last month and deployed his units to defend the Sanaa protesters, called on "all honest people" to rise up against what he called the killers of the regime.

He denounced Wednesday's killings and said in a statement that Saleh was trying to sow divisions within the armed forces. He did not elaborate. Al-Ahmar was the most senior army commander to defect to the side of the protesters. Others included lawmakers, politicians, Cabinet ministers, policemen and diplomats.