At least 22 people have died in clashes between Arab and Berber communities around the Algerian oasis city of Ghardaia, the state news agency says.
The city has seen clashes for the last two years, with rivalry among communities for jobs, housing and land.
But the two days of clashes in Ghardaia and two nearby cities - Guerrara and Berianne - are the most violent yet.
President Abdelaziz Bouteflika convened an emergency meeting on Wednesday.
A statement by the president urged people in Ghardaia "to help to restore calm and to preserve the age-old bonds of brotherhood which have always marked the region".
Interior Minister Noureddine Bedoui arrived in Ghardaia, a Unesco world heritage site, earlier in the day.
Mozabites - members of the local Berber community - called for better protection in a protest in the capital, Algiers, 600km (373 miles) to the north.
"The situation is very serious," one Mozabite leader told AFP. "This is not just clashes any more, it's terrorism."
Algeria's official news agency, APS, said most of the deaths were from "projectiles". It is not known to which groups the dead belonged.
A cemetery belonging to Mozabites was desecrated in December 2013, leading to fighting between groups. At least a dozen people had been killed before violence flared up again this week, AP said.