A suicide bomber killed at least 42 people on Thursday when he detonated an explosives-laden belt in a district of the Yemeni capital where the powerful Shi'ite Muslim Houthi movement had planned to hold a rally, medics and witnesses said.

A Reuters witness in Sanaa counted at least 20 bodies immediately after the attack on a checkpoint held by the Houthis, the country's main power brokers since their paramilitary forces seized the capital on Sept. 21 following weeks of anti-government demonstrations.

Medical sources later said the death toll had risen to at least 42, including several children. The death toll was expected to climb further, with many of those wounded in serious or critical condition, they said.

In a separate incident, at least 20 government soldiers were killed in a suicide car bombing and gun attack in the country's east on Thursday, state news agency SABA reported.

The attacks occurred just hours after a political showdown between the Houthis and President Abd-Rabbu Mansour led to the resignation of Prime Minister Ahmed Awad bin Mubarak, whose appointment on Tuesday had angered Houthi leaders.

A policeman guarding a local bank near Tahrir Square in central Sanaa, said a man apparently wearing a suicide belt approached the Houthi checkpoint.

"He then exploded amidst the (Houthi) security and ordinary people nearby," the policemen told Reuters.

In Buroom, a coastal region of the eastern Hadramout province, a suicide bomber drove a car laden with explosives towards an army camp, while gunmen tried to storm the facility, a local official and witnesses said. The soldiers beat back the attackers, but SABA said 20 of the troops were killed.

No one has claimed responsibility for the attacks, but the incidents appear to mirror previous bombings carried out by Al Qaeda in the Arabian Peninsula (AQAP), which has targeted state institutions, including the armed forces, and which sees members of the minority Zaydi sect of Shi'ite Islam as heretics.

Western and Gulf Arab countries are worried that instability in Yemen could strengthen al Qaeda and have supported a U.N.-backed political transition since 2012 led by Hadi meant to shepherd the country to stability after decades of autocracy.

PROTEST

A new Yemeni government is due to be appointed under a power-sharing accord signed last month aimed at bringing the Houthis into government. Once a new administration is nominated the Houthis are meant to withdraw their forces from the city, allowing the army and police to resume their duties.

The Houthis on Wednesday rejected Hadi's nomination of bin Mubarak as prime minister, and bin Mubarak announced early on Thursday he had agreed not to take up the position.

Houthi followers had been preparing to demonstrate in Tahrir Square on Thursday to voice opposition to the nomination of bin Mubarak, previously the head of Hadi's office, on the grounds that his selection had been imposed by Washington. The United States has denied the allegation.

The Houthis said the protest would proceed despite the attack, and thousands of supporters, some armed, converged on the square chanting slogans against the government and corruption.

"This terrorist attack would not deter us from holding this demonstration," a local organiser told Reuters.

"UNACCEPTABLE MISTAKE"

The Houthis said they had foiled another attack by two cars on the square earlier in the morning, destroying one vehicle, while attackers in a second car managed to escape.

The Houthi capture of Sanaa has alarmed Saudi Arabia, the world's top oil exporter which shares a long border with Yemen, and other conservative Gulf Arab states.

On Wednesday evening Houthi leader Abdulmalik al-Houthi called for mass protests against "foreign interference" he said was behind the appointment of bin Mubarak.

"I assert that together with these marches tomorrow, God willing there will be important steps that will contribute to correcting this mistake, which is an unacceptable mistake," Houthi said.

SABA said that Hadi accepted bin Mubarak's decision to turn down the appointment and resumed consultations to agree on a new prime minister.