A bomb blast tore through Tahrir Square in central Sana’a on Thursday morning, killing dozens of supporters of the Houthi movement that controls the Yemeni capital and fuelling fears of more violence between the Shia group and radical Sunni Islamists.

The explosion underlined the mounting political and security vacuum in Sana’a, which came under Houthi control on 21 September shortly before a peace deal was signed between the group and the government.

The health ministry estimated that the attack left at least 47 people dead and 75 wounded, with the death toll likely to rise because a number of survivors are critically injured.

The suicide bomber detonated the device as Houthi supporters gathered for a rally in the square. An hour after the explosion blood still stained the ground near the blast site and a pile of body parts lay in the open, discarded by paramedics speeding to treat survivors.

Government officials and analysts believe the attack was the work of al-Qaida in the Arabian Peninsula (AQAP), which seized a major military compound in Sana’a in December 2013.

In September AQAP declared war on the Houthis, whose manifesto includes revival of the Zaydi form of Shia Islam almost unique to the north of Yemen. The Houthis have vowed to eradicate their Sunni rivals, but they have blamed Washington for the Tahrir bombing, claiming the attack was part of a campaign of foreign intervention to destabilise in the country.

“The Americans did this,” said Abdullah Youssef, a Houthi supporter who stood guard at the scene of the bombing shortly after the explosion, in what amounts to an escalation of the group’s anti-US rhetoric. Over the wail of sirens and chanting from the crowd gathered in the square, Youssef said that “Da’ash” – a slang term for Islamic State, that many Yemenis use as a catch-all term for al-Qaida – and the US government were “the same”.

Such fiery rhetoric has been growing among Houthi supporters since their takeover of Sana’a. The night before the blast, the movement’s leader, Abdulmalik al-Houthi, a hardline critic of foreign intervention particularly from the US, accused the newly appointed prime minister, Ahmed Awad bin Mubarak, of being an “agent of America” and demanded he step down. He also accused Yemen’s president, Abd Rabbu Mansour Hadi, of being a puppet of foreign powers, and ignoring the terms of the 21 September peace deal which called for broad political consensus on the appointment of a new prime minister.

“Ansar Allah are promoting heavy anti-US rhetoric,” a government official told the Guardian, using the group’s preferred name, which translates as Partisans of God. Mubarak, who was appointed on Tuesday, announced early on Thursday that he had agreed not to take up the position following Houthi opposition to his taking office. His resignation worsened a worrying political vacuum after two weeks of failed negotiations to form a new government, and put further pressure on Hadi.

The president’s appointment of his chief-of-staff as prime minister was a “serious political mistake” said April Alley, Arabian peninsula analyst at International Crisis Group. A number of other political groups had also rejected Mubarak’s appointment, she added.

The Tahrir bombing was the latest in a series of AQAP attacks in recent weeks, and many in the capital fear that it will open the floodgates for widespread bloodletting, with the Houthis publicly vowing to attack the group in its southern strongholds. “Violence will most likely intensify and blood will be spilled,” the Yemeni official said. “It seems we are not immune from regional trends after all.”