At least 63 people have been killed in a suicide bomb attack during a rehearsal for a military parade in the Yemeni capital, Sanaa, officials say.
The assailant, who was reportedly wearing army uniform, blew himself up among a group of soldiers at al-Sabin Square, near the presidential palace.
One unconfirmed report said as many as 96 people might have been killed.
It was the deadliest attack in the capital since President Abdrabbuh Mansour Hadi took power in February.
Witnesses said remains of the victims were scattered across the square, where large military parades are often held. Ambulances rushed to the area to take the dozens of wounded to hospital.
"We were in a parade, suddenly there was a huge explosion. Dozens of our men were killed. We tried to help them," Col Amin al-Alghabati told the Reuters news agency.
"The suicide bomber was dressed in a military uniform. He had a belt of explosives underneath," he added.
It is not clear if the attacker had been taking part in the drill or had walked up to the soldiers shortly before detonating his bomb.
Defence Minister Nasser Ahmed and the army chief-of-staff were reportedly in the square at the time, but were not hurt.
Medical sources told the AFP news agency that at least 96 soldiers had been killed and 300 wounded, who they said were being treated in seven hospital across Sanaa.
No group has said it was behind the bombing, but several soldiers blamed al-Qaeda.
They had been practising for a parade for National Unity Day on Tuesday, which marks the anniversary of the 1990 unification of the Marxist People's Democratic Republic of Yemen - also known as South Yemen - and the Yemen Arab Republic - known as North Yemen.
"Yemenis must stand together in the face of this deadly terrorist threat," Brig Karim Nahil said. "We will celebrate our unity tomorrow with the blood of our martyrs on our hands and faces."
Monday's attack comes 10 days after the military launched an offensive against Islamist militants linked to al-Qaeda in the Arabian Peninsula (AQAP) in the southern province of Abyan.
Over the weekend, at least 33 militants and 19 soldiers were reportedly killed in clashes near the town of Jaar, in Abyan, which has been under control of Ansar al-Sharia - an offshoot of AQAP - since last year.
On Sunday, a US Coast Guard instructor was shot and wounded by unidentified gunmen as he drove through the eastern Red Sea port of Hodeida. Ansar al-Sharia later said it had been behind the attack.
Ansar al-Sharia, or Partisans of Islamic law, was founded in response to the growing youth movement in Yemen, which has marginalised Salafi jihadists who advocate the violent overthrow of the government.
In November, protesters forced President Ali Abdullah Saleh to hand over power to Mr Hadi, then his vice-president, as part of a deal brokered by the Gulf Co-operation Council.
Mr Hadi was elected president in February and immediately said one of his most important tasks was the "continuation of war against al-Qaeda as a religious and national duty".
The president has also moved to restructure the military, trying to purge it it of those closest to his predecessor.