The political crisis in Yemen deepened on Thursday as the resignation of President Abd-Rabbu Mansour Hadi plunged the country into a power vacuum, Reuters reported.

Hadi had been held as a virtual prisoner at his home residence after Shiite rebels invaded the Yemeni capital of Sanaa earlier this week. But his resignation apparently caught the militants, known as Houthis, off guard, the British news service added.

The outgoing president blamed the rebels for impeding his attempt to steer Yemen, the poorest country in the Arab world, toward increased stability. The nation has seen years of turmoil and tribal unrest, and U.S. drone strikes frequently targeted local militants.

The Houthis, meanwhile, battle on two fronts - opposing both Hadi's central government and the Yemeni offshoot of al-Qaida, known as "al-Qaida in the Arabian Peninsula," which is considered the most dangerous branch of the global terror network.

Supporters and detractors of the Shiite group that occupied Sanaa in September, meanwhile, took to the streets in the Yemeni capital. Thousands gathered in the heart of the city with placards calling for "Death to America, Death to Israel", a slogan that has become a Houthi trademark, Reuters noted.

"Hadi should have resigned a long time ago," said Al Sheikh Moghadal Al Wazeer, an elderly supporter. "He should have done more, and he should have run the country with more strength."

A small group of pro-democracy activists, though, marched on Change Square, the focus of 2011 protests that forced longtime President Ali Abdullah Saleh from power. In Aden, protesters raised the flag of the formerly independent South Yemen over the city's airport and the security headquarters building, witnesses told the Associated Press.

Houthi officials welcomed Hadi's resignation but said an official position had yet to be taken, Reuters noted. The rebels urged the army to uphold its duties.

Critics in the United States judged that "the collapse of the Yemeni government ... has raised doubts about the efficacy of (President Obama's) 'light footprint' counterterrorism strategy," CBS News said.

The feared power vacuum creates an opening for "al-Qaida in the Arabian Peninsula," which recently claimed responsibility for the attack on the French satiric magazine Charlie Hebdo and was also behind several foiled plots against U.S. targets.