A suicide bomber infiltrated a rehearsal of a military parade in Sanaa yesterday, killing at least 90 people, and wounding a further 200, mostly soldiers.
It was the bloodiest single attack in the Yemeni capital during the country's recent years of instability.
Yemen's defence minister and chief of staff were both present at the event, a rehearsal for today's National Day parade, but neither was hurt.
Militant group Ansar al Sharia claimed responsibility. A spokesman said it was in response to the "crimes" of the Yemeni security forces who are fighting to dislodge militants from their strongholds in the south.
The authenticity of the claim could not immediately be verified, however, and officials suspect the bombing was carried out by a rogue soldier.
The attack has heightened ten-sions, already high, in Washington. Yemen is a front-line state in the US war on Islamic militants.
The day before the Sanaa bombing, a US military instructor was shot and seriously wounded in an attack also claimed by Ansar al Sharia, which is affiliated to al Qaeda in the Arabian Peninsula.
Militants, such as Ansar al Sharia, have exploited Yemen's political instability to gain a strategic foothold in the country, which was paralysed for most of 2011 by the protests that unseated President Ali Abdullah Saleh.
Yesterday's huge explosion left scenes of carnage, with bloodied victims strewn across the 10-lane road where the rehearsal was taking place, not far from the presidential palace.
"We had just finished the parade. We were saluting our commander when a huge explosion went off," said soldier Amr Habib.
"It was a gruesome attack. Many soldiers were killed and others had their arms and legs blown off."
One of the investigators said preliminary findings suggested the suicide bomber was a rogue soldier rather than a man wearing a disguise. Normal security procedure for such an event is for checks to be made on soldiers at their bases prior to their transportation, in army vehicles, to the parade ground.
Colonel Amin al Alghabati, his hands and uniform flecked with blood, said: "The suicide bomber was dressed in a military uniform. He had a belt of explosives underneath."
As many of the wounded were ferried to hospital in taxis, Dr Mohsen al Dhahari said Sanaa's hospitals were in danger of being overwhelmed by the number of casualties.
He added: "Most of the injuries are to the head, we have dozens paralysed. We expect the death toll to rise. Most of the injured here are boys in their teens."
The impoverished state has seen a spate of deadly attacks since President Abd-Rabbu Mansour Hadi replaced Mr Saleh in February. Mr Hadi said he would extinguish an Islamist insurgency, which until now has been concentrated in the south.
On Sunday, Yemeni troops closed in on the southern militant-held town of Jaar in heavy fighting, part of a US-backed offensive launched this month to regain control of territory and towns seized by Ansar al Sharia.
The parade, scheduled for today, was to mark the unification of north and south Yemen, previously separate states, which were merged in 1990.
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