Hundreds of Muslims have protested in Mauritania's capital, Nouakchott, after news spread that the Koran had allegedly been desecrated at a mosque.
An imam reported that four men with their faces covered by turbans entered the mosque, tearing copies of the Koran and throwing them into a toilet.
Police fired tear gas to disperse angry crowds, and one person was killed, a hospital source said.
Mauritania is a Muslim country with a tiny Christian population.
It has been a key ally of the West in the campaign against al-Qaeda and other militant groups operating in the region.
Journalist Hamdi Mohamed El Hacen in Nouakchott says protesters burnt barricades and many shops and markets remained closed on Monday.
Hundreds of protesters gathered in the city centre and outside various mosques, chanting "God is Great" and demanding that the men be arrested and given the death sentence for allegedly blaspheming Islam, he says.
A hospital source told Reuters news agency that one person was killed after apparently being hit by an exploding tear gas canister.
It is unclear who allegedly desecrated the Koran or what their motive was.
The imam, Mohamedoun Ould Mohamed Salem, said the men took four copies of the Koran, which they then shredded and threw into the mosque's toilet.
Public anger has been compounded by two recent incidents of alleged blasphemy, Mr El Hacen says.
In February, a man was arrested and sent for psychiatric evaluation after urinating on a Koran in northern Mauritania.
A young Muslim is facing a possible death sentence after being convicted in January of apostasy and jailed for writing an article criticising the Prophet Mohammed.
An Islamist party, Tawassoul, became the main opposition party in Mauritania in December after polling the second highest number of votes in legislative elections.