Sokoto, Nigeria - Police have fired teargas to disperse rival Muslim groups in the northern Nigerian city of Sokoto, home to the country's most senior Muslim leader.
A Shia group held a march to mark the birthday of the Prophet Mohammed, despite warnings not to.
Members of the rival Sunni community, who form the majority in Sokoto, then pelted the marchers with stones, says the BBC's Ibrahim Dosara in the city.
In February, three people were killed in clashes between the groups.
There were no reports of serious injuries in Wednesday's clashes but 21 people were arrested.
As well as throwing stones, the Sunnis set up burning barricades in several parts of the city.
Sokoto, home to the Sultan of Sokoto, is seen as the historic centre of Islam in Nigeria.
Nigeria's 130 million people are roughly equally divided between Muslims in the north and Christians in the south.
Correspondents say that poverty drives rival groups to compete for scarce resources, such as land.