A band of Al Qaeda militants seized full control of a town 100 miles south of the Yemeni capital Sanaa on Monday, overrunning army positions, storming the local prison and freeing at least 150 inmates, security officials said.

The capture of Radda expanded already significant territorial conquests by the militants, who have taken advantage of the weak central government and political turmoil roiling the nation for the past year during an uprising inspired by Arab Spring revolts. Authoritarian President Ali Abdullah Saleh recently agreed to step down, but he remains a powerful force within the country and a spark for ongoing unrest.

The group had previously taken control of a string of towns in the mostly lawless south. But its capture of Radda is particularly important because it gives the militants a territorial foothold closer than ever before to the capital, where many sleeper cells of the terror network are thought to be located.

An Associated Press photographer who visited Radda on Sunday said the militants were armed with rocket-propelled grenades, automatic rifles and other weapons. He quoted residents as saying the black Al Qaeda banner has been raised atop the mosque they captured over the weekend.

The opposition accused Saleh, who is to step down this month in line with a power transfer deal, of allowing the militants to overrun Radda along with two other towns in southern Abyan province captured previously -- Zinjibar and Jaar -- to bolster his claims that he must remain in power to secure the country against the rising power of Islamist militants.

Some tribal leaders also accused Saleh of giving the "green light" to the militants to overrun the city.

"We are surprised by the silence of the security forces," said opposition activist Abdel-Rahman al-Rashid, who lives in Radda. "They have not moved, which only means that this is all arranged to spark chaos."

The United States and its western and Gulf Arab allies such as Saudi Arabia long considered Saleh a pivotal ally in the fight against Yemen's active Al Qaeda branch, which has been linked to terror attacks on U.S. soil and is believed to be one of the international terror organization's most dangerous franchises. The U.S. withdrew its support last summer and said Saleh should step down.

According to security officials, a band of about 200 militants pushed into Radda on Monday from several points they had captured over the weekend, including an ancient castle that overlooks the town, a school and a mosque. They stormed the local jail and freed 150-200 inmates, including an unspecified number of militants loyal to Al Qaeda.

Some of the freed inmates joined the militants after they were given arms, according to the officials who spoke on condition of anonymity because they were not authorized to speak with the media.

The officials said the Al Qaeda fighters were led by Tariq Al-Zahab, a Yemeni whose sister was married to U.S.-born Al Qaeda cleric Anwar al-Awlaki, killed in a U.S. airstrike last September.

The fighters later threw up a security ring around Radda, preventing residents from leaving or entering, and killed two soldiers and wounded a third in clashes with army troops. They also seized weapon caches and vehicles from the security headquarters.

In midday, clashes erupted between Al Qaeda militants and armed tribesmen, leaving one dead and two wounded, according to a member of al-Qayfa tribe which took part in the clashes. He also spoke on condition of anonymity because of security concerns.

Monday's attack prompted stores and schools in Radda to close. Thousands demonstrated in the provincial capital of Bayda to protest the perceived failure of security forces to protect the town, which has a population of about 40,000.

Radda is part of Bayda province, a key transit route between the capital and Yemen's southern provinces where Al Qaeda-linked militants have already seized control of a swath of territory and towns in Abyan province. Radda is only about 16 miles away from a main road that links Sanaa to eight provinces, raising the specter that taking the town could be a prelude to isolating the capital.

Al Qaeda-linked militants began seizing territory in the southern Abyan province last spring, solidifying their control over the town of Jaar in April before taking the provincial capital, Zinjibar, in May. Abyan borders Bayda.

Yemeni security forces have been trying unsuccessfully to push them out since then in fierce fighting that has caused many casualties on both sides. The conflict has forced tens of thousands of civilians from Zinjibar and the surrounding area to flee, many to the port city of Aden.