Forces loyal to embattled President Ali Abdullah Saleh have opened fire on tens of thousands of protesters in Yemen's capital, Sanaa, killing at least 18 and wounding scores, according to medical officials and witnesses.

Witnesses said forces loyal to Saleh fired on the protesters with assault rifles and anti-aircraft guns Saturday while video from the scene showed water cannons being used to drive back those in the crowd.

Gunfire could be heard in the area of the clashes well into the afternoon.

Medical officials said at least 180 people were wounded. The wounded were ferried to hospital in ambulances and on motorbikes.

People in Yemen's capital have been calling for the resignation of Saleh and for regime change in the country since the spring.

Elsewhere in Yemen, the oldest son of slain U.S.-born cleric Anwar al-Awlaki was killed late Friday, along with eight other al-Qaeda militants, including the media chief of the terror group's Yemeni branch, according to government officials.

The Yemeni Defence Ministry said its air force targeted militant hideouts near the town of Azzan in the province of Shabwa, killing al-Awlaki's son and a cousin. The son was identified as 21-year-old Abdul-Rahman al-Awlaki..

Initial reports quoted tribal elders as saying they believed the aircraft used in the air strikes were foreign.

Anwar al-Awlaki, an American-born cleric, was once a prominent imam at mosques in Virginia and San Diego, Calif.

The older al-Awlaki, a Muslim preacher and savvy Internet operator who has been described as a powerful al-Qaeda recruiting tool, was killed Sept. 30, also in a U.S. drone attack.

The Yemeni Defence Ministry identified the slain media chief as Egyptian-born Ibrahim al-Banna.

The first strike late Friday targeted a house in the Azan district of Shabwa, but hit just after al-Qaeda militants had a meeting in the building, security officials and tribal elders said.

Militant group active as political turmoil continues

They said a second strike then targeted two sport utility vehicles in which al-Banna and a number of militants were travelling, destroying the vehicles and leaving the men's bodies charred. It was not clear whether other participants in the meeting were targeted in separate strikes.

AQAP has taken advantage of the political turmoil roiling Yemen.

Saleh, who has ruled the country for more than 30 years, has been struggling to stay in power in the face of eight months of massive street protests demanding his ouster and the defection to the opposition of key aides and military commanders.

Militants linked to AQAP have taken over several cities in the south, raising fears that they could establish a permanent stronghold in this strategically located nation. Yemen is located at the southern tip of the Arabian Peninsula, on the doorstep of Saudi Arabia and the oil-producing nations of the Gulf. It also overlooks strategic sea routes leading to the Suez Canal.

In a separate development, the security officials said suspected al-Qaeda militants bombed a key underground gas pipeline that extends from the Balhaf area in Shabwa to an export terminal on the Arabian Sea. The Friday night attack started a massive fire, with columns of flames illuminating the night sky.

The security officials said non-Yemeni employees of the French company running the gas field and pipeline in Balhaf have been evacuated to Sanaa aboard three helicopters for their safety. They had no more details.