N'DJAMENA, Chad -- A man disguised as a woman wearing a burqa detonated a bomb outside the main market in Chad's capital Saturday morning, killing 14 people, a police spokesman said. It was the third attack in N'Djamena in less than a month -- a spate of violence widely believed to have been organized by Nigeria-based Islamic extremists.

"After the explosion, the provisional toll is 15 dead including the bomber," police spokesman Paul Manga said.

The victims included five men and nine women who worked at the market, Manga said, adding that 74 people were injured.

The bomber attempted to enter the market, then detonated his bomb when military police tried to stop him, Manga said. A witness at the scene, Esaie Maloum, said he overheard the bomber praying and saying "Allahu Akbar," or "God is great" in Arabic, before carrying out the attack.

Boko Haram this year became the West African franchise of the Islamic State group, which has called for more attacks during the holy month of Ramadan.

Saturday's blast came less than a month after suicide bombers on motorcycles attacked two buildings in N'Djamena, killing nearly 40 people in the first assault of its kind since the Islamic militants threatened the country earlier this year.

After that attack, Chad announced a ban on women wearing the burqa in an attempt to prevent suicide attacks, though the measure has not been strictly enforced.

In late June, two more suicide bombings in N'Djamena killed 11 people including five police officers.

Boko Haram has also targeted Chadian villages along the lake dividing the two countries.

Chad has been a major participant in the fight against Boko Haram, which has publicly threatened Chad President Idriss Deby with retaliation.