Al-Qaeda has claimed responsibility for a suicide bombing in the centre of the Yemeni capital that has left nearly 100 people dead.
Officials have said a bomber dressed in military uniform targeted soldiers rehearsing for a parade in Sanaa to mark Yemen's National Day.
Yemen's defence minister and chief of staff were both present at the event but neither was hurt.
Al Jazeera's Jane Ferguson, reporting from Sanaa, said that the death toll was still climbing by late Monday afternoon.
"We are hearing reports that 96 people were killed and many more injured," our correspondent said. "There have been requests for blood donations and the death toll could go even higher."
The huge explosion left scenes of carnage at Sabaeen Square, with bloodied victims strewn across the 10-lane road where the rehearsal was held on Monday morning 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."
Yemeni President Abd-Rabbu Mansour Hadi is scheduled to attend Tuesday's parade marking the 22nd anniversary of the unification of north and south Yemen.
UN Secretary General Ban Ki-moon strongly condemned the attack, saying that those behind the attack must be brought to justice.
"The secretary general calls on all in Yemen to reject the use of violence in all its forms and manifestations, and expects them (to) play a full and constructive role in implementing Yemen's political transition agreement," Ban said in a written statement.
Al-Qaeda's wing in Yemen said the suicide bombing was in revenge for what it called the US war on its followers in southern Yemen and that it had targeted the Yemeni military brass.
The group also warned in a statement received by the Reuters news agency that more attacks will follow if the military campaign in the southern province of Abyan did not stop.
"We will take revenge, God willing, and the flames of war will reach you everywhere, and what happened is but the start of a jihad project in defence of honour and sanctities," the statement said.
But one of the government investigators said preliminary findings suggested the suicide bomber was a rogue soldier rather than a man in a disguise.
"The suicide bomber was dressed in a military uniform. He had a belt of explosives underneath," said a man who identified himself as Colonel Amin al-Alghabati, his hands and uniform flecked with blood.
A US military instructor was shot and seriously wounded on Sunday in an attack also claimed by Ansar al-Sharia, which is affiliated with Al Qaeda in the Arabian Peninsula (AQAP).
Yemeni government soldiers are waging a fierce campaign in the country's south against AQAP fighters who have taken advantage of political instability to gain territory.
Yemeni troops closed in on the southern militant-held town of Jaar on Sunday in heavy fighting, part of a new US-backed offensive launched earlier this month to regain control of territory and towns seized by Ansar al-Sharia.
Our correspondent said the blast appeared to have been intended to send a signal to the government and the military.
"If they were targeting high-profile guests, like the president at the parade, then they would have attacked tomorrow," she said.
"This was a message to the authorities that they are willing and able to strike at the heart of the military and that nobody here is safe."