SANAA, Yemen — A suicide bomber struck at the center of the Yemeni capital of Sanaa on Thursday, setting off his explosives at a gathering of supporters of the rebel Shiite Houthis who recently overran the city, security officials said.

The massive blast reverberated through the heart of Sanna but the number of casualties was not immediately known. A television channel that supports the rebels, al-Maseera, said that at least 32 people died in the explosion but the report could not be immediately confirmed.

The Al-Arabiya satellite channel broadcast footage from the immediate aftermath, showing several lifeless bodies lying on the street and large pools of blood on a sidewalk.

Yemeni officials could not immediately be reached for comment. Security officials told The Associated Press that the explosion was the work of a suicide bomber, but provided no further details. They spoke on condition of anonymity because they were not authorized to talk to media.

The attack, however, bore all the hallmarks of al-Qaida, whose Yemeni branch is considered by Washington to be the world's most dangerous offshoot of the terror network. It has also staged large-scale attacks and bombings in the past.

Hawthi Shiite protesters carry the coffin of one of their fellow protesters who were killed in recent clashes with Yemeni police, during their funeral procession in Sanaa, Yemen, Friday, Sept. 12, 2014. Image: Hani Mohammed/Associated Press

Last week, al-Qaida in Yemen warned it would target the Houthis and called on the country's Sunnis to close ranks and fight the Shiite rebels.

Thursday's blast took place as hundreds of Houthi supporters began to gather on the city's central Tahrir Street for a demonstration that had been called to protest President Abed Rabbo Mansour Hadi's choice for new prime minister, Ahmed Awad bin Mubarak.

A Houthi Shiite rebel holds a weapon while on patrol following dawn prayers attended by supporters of the rebels on the first day of the Muslim holiday of Eid al-Adha, or Feast of the Sacrifice, in Sanaa, Yemen, Saturday, Oct. 4, 2014 Image: Hani Mohammed/Associated Press

As the crisis escalated, the prime minister-designate asked Hadi early on Thursday to relieve him from the post.

But the Houthi rally — despite the suicide bombing and bin Mubarak declining the premiership — went on later Thursday, with some 4,000 Houthis calling on Hadi to step down and chanting slogans against the United States and Saudi Arabia.

Rebel leader Abdel-Malik al-Houthi delivered a televised statement on Wednesday night calling on supporters to rally Thursday against the choice of bin Mubarak. He said his group was surprised by the nomination, saying it came after Hadi met with the U.S. ambassador to Yemen. Al-Houthi called Hadi a "puppet" in the hands of foreign powers.

"Blatant foreign interference is a form of circumventing the popular revolution," he said.

A Hawthi Shiite rebel gestures from a tank at the compound of the army's First Armored Division, after they took it over, in Sanaa, Yemen, Monday, Sept. 22, 2014.

The Houthis took control of the capital last month, the same a U.N.-brokered deal was reached, ending fighting in the capital. The Houthi takeover of Sanaa followed weeks of protests by their supporters in the capital to press demands for a larger share in power and a change in government.

The Sept. 21 deal called for the appointment of a new head of government, and for armed Houthis to pull out of the city.

Bin Mubarak, 47, was the head of the president's office. Previously, he had successfully led an effort by various political parties — including longtime rivals — to devise a political map for transition after a 2011 uprising in Yemen, an impoverished country in the southern corner of the Arabian Peninsula.

He is one of the youngest politicians in Yemen. He emerged during the uprising that erupted in 2011 and the following year forced longtime leader Ali Abdullah Saleh to step down as part of a U.S. and Saudi-backed accord. Saleh handed over to Hadi but continued to wield significant power.