SANAA, Yemen (AP) -- Yemeni troops loyal to embattled President Ali Abdullah Saleh opened fire Tuesday at thousands of protesters calling for his ouster in the capital Sanaa, killing two, a medical official said.

The protesters marched through the streets surrounding Change Square, a central intersection where the uprising against Saleh started in February.

"The people want to prosecute the butcher,'' the protesters chanted, and some held posters saying that after the death of Libya's Moammar Gadhafi, it was time for Saleh to ``listen to your people.''

The shooting broke out between Saleh's forces and renegade troops loyal to Maj. Gen. Ali Mohsen al-Ahmar, who defected to the opposition and whose forces protect the protesters.

Mohammed al-Qubati, who runs a field hospital for the protesters, said two protesters were killed and at least 40 were wounded in the shooting. He said dozens had breathing difficulties from tear gas fired by the troops.

Saleh has clung to power despite more than nine months of massive street protests against him, inspired by Arab uprisings. After a June assassination attempt, he went to Saudi Arabia for treatment but abruptly returned to Yemen last month.

Saleh has also balked at signing a deal brokered by his powerful Arab Gulf neighbors and the United States in hopes of providing a smooth transition of power. Under the deal, Saleh would resign and hand power to his vice president in return for immunity from prosecution.

There are also worries that the intensified fighting could undermine U.S. and Saudi efforts to fight Yemen's al-Qaida branch, considered by the U.S. to be the most dangerous of the terror network's affiliates after it plotted two recent failed attacks on American soil.

On Friday, a U.N. Security Council resolution called for Saleh to immediately accept the deal and expressed grave concern at the situation in Yemen.

Also Tuesday, a Yemeni military plane crashed shortly before landing at the al-Ammad air base near the southern city of Aden.

Four people on board were killed and 11 injured, according to a security official who spoke on condition of anonymity because he was not authorized to talk to the media. The official said a technical problem might have caused the crash. He said there were eight Syrians and seven Yemenis on board.