(CNN) -- At least 121 people were killed and 45 injured in an explosion at an ammunition factory in southern Yemen on Monday, medical sources said.

The death toll was expected to rise, said the sources, who asked that they not be identified because they are not authorized to speak to the news media. Two of them work at Republican Hospital in Abyan.

Most of the dead and injured were locals who had been ransacking the factory after it was taken over Sunday by militants, security officials said.

The explosion took place in Abyan Province, they said.

It comes after months of demonstrations against Yemen's President Ali Abdullah Saleh, and a weekend of violence between government forces and the local al Qaeda group.

Fighting over the past two days between Yemeni security forces and members of al Qaeda in the Arabian Peninsula left people on both sides dead, Yemeni security forces said Sunday.

Saleh has been fighting to hold onto power, arguing that he is best equipped to lead the fight against Islamists.

Three "al Qaeda terrorists were killed" and six others were arrested in Lawdar district, Yemen's official news agency Saba reported Saturday.

Also Sunday, seven Yemeni soldiers were killed and seven others were wounded when members of al Qaeda in the Arabian Peninsula attacked them in Marib, two security officials said. The attack took place at a military checkpoint a mile north of the government complex in Marib Province, east of the capital, Sanaa.

The attackers used grenades and machine guns, and took two military vehicles from the checkpoint, the security forces said.

After the fighting Saturday, the Yemeni government said it was a sign that strong measures are needed to combat instability within the country. Saba reported that Governor Saleh al-Zawari of Abyan Province affirmed "the importance to enhance security performance to strictly confront any attempts to disturb security and stability in the governorate."

Yemen has been facing protests from people citing government corruption, a lack of political freedom and high unemployment.

Calls for Saleh's ouster have increased in recent weeks following revolutions in Egypt and Tunisia. Saleh has ruled since 1978.

Saleh said Sunday he will not offer any more concessions, and he described the opposition as an alliance against the country's majority, according to Saba.

"I derive my strength from the people, not from the tank," Saleh was quoted as telling a session of the General People's Congress Standing Committee.

Opposition leaders rejected the president's comments. "Saleh has not learned a lesson from the millions who are demanding he step down," said Yaseen Noman, president of the JMP, the country's largest opposition bloc. "All the different initiatives and negotiations that take place are only there for Saleh to buy himself more time," he said.

"Saleh's lies show the world that he must leave power -- and people understand this," Noman added. "Our demands are simple: He must leave office. This is not open for negotiation."

The country has been wracked by a Shiite Muslim uprising, a U.S.-aided crackdown on al Qaeda operatives and a looming shortage of water.

Saleh has been a staunch U.S. ally in the fight against al Qaeda in the Arabian Peninsula.

U.S. Defense Secretary Robert Gates was asked last week about how the United States sees cooperation with Yemen if Saleh were to step down.

"We have had a good working relationship with President Saleh," Gates told reporters during a trip to Cairo. "He's been an important ally in the counterterrorism arena. But clearly, there's a lot of unhappiness inside Yemen. And I think we will basically just continue to watch the situation. We haven't done any post-Saleh planning, if you will."

Saleh has said he accepts opposition demands for constitutional reforms and holding parliamentary elections by the end of the year.

Saleh has also promised not to run for president in the next round of elections.

CNN's Mohammed Jamjoom and journalist Hakim Almasmari contributed to this report.