Medics told AFP that 49 people were killed and 70 wounded in Wednesday's attack carried out by a bomber dressed as a woman in the mainly Sunni city of Ibb, held by the Huthi Shiite militia.
An initial toll gave at least 33 dead and dozens wounded.
Yemen has been dogged by instability since an uprising forced longtime strongman Ali Abdullah Saleh from power in 2012, with Huthi rebels and Al-Qaeda seeking to fill the power vacuum.
Among those wounded in Wednesday's attack was Ibb governor Yahya al-Aryani, who was attending the Huthi commemoration of the Prophet Mohammed's birthday, a medic said.
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A security source told AFP the bomber had been wearing a woman's black abaya and veil when he detonated his suicide belt.
The United States, allied with Yemen in fighting militants, "strongly condemns" the attack, a State Department spokesman said.
"We will continue to stand with the government of Yemen and the Yemeni people to counter the shared threat posed by violent extremists," added Jeff Rathke, the spokesman.
Al-Qaeda in the Arabian Peninsula, the Yemen branch of the jihadist organisation, has pledged to fight the Huthis, who overran the capital Sanaa unopposed in September and have since advanced into mainly Sunni districts such as Ibb.
The US slapped sanctions on former president Saleh in November for allegedly supporting the Huthi advance.
It also blacklisted two Shiite rebel commanders, Abdullah Yahya al-Hakim and Abd al-Khaliq al-Huthi, "for engaging in acts that directly or indirectly threaten the peace, security, or stability of Yemen."