At least 10 people died in the blast, and dozens more were injured, including seven critically after a suicide bomber detonated his explosives outside a police academy in the Yemen capital Sanaa, a security official said.
Ambulances could be seen rushing to the site of the attack in the capital Sanaa. Dozens more were wounded, including several critically.
There was no immediate claim of responsibility for the blast, but al Qaida's Yemen branch frequently targets security forces.
Twelve suspects have been arrested. The bomber was from the province of Amran, about north-west of Sanaa.
The capital was on high alert after the attack, with security forces setting up checkpoints around the city and searching cars. Security was also beefed up around embassies.
The attack came after the army last month recaptured several militant-held towns in the country's south, following a months-long campaign to retake territory the militants seized during last year's political turmoil that swept the country.
Al Qaida in the Arabian Peninsula, considered the global terror movement's most dangerous offshoot, has struck back against the military's offensive with deadly attacks in the south and a May 21 bombing at a parade ground in Sanaa that killed 96 Yemeni soldiers.
Security officials said 55 people have been arrested in connection with that attack, among them al Qaida militants accused of plotting to attack the US Embassy.
Earlier today, the government announced that two al Qaida militants who tunnelled out of a prison last month were re-arrested in a southern province. One of the two, Nasser Ismail Ahmed Muttahar, was detained for taking part in an attack on the US Embassy in Sanaa in 2008.
The attack on the embassy's gate, carried out by gunmen and vehicles packed with explosives, killed 19 people including an 18-year-old American woman and six militants. None of those killed or wounded were US diplomats or embassy employees.
In another of Yemen's multiple ongoing conflicts, the army shot dead a protester today in the southern port city of Aden and wounded four others, including two women.