Security forces arrested 21 separatists trying to provoke rioting during demonstrations in a southern provincial capital as Yemen increased security to guard against attacks, the government said on Sunday.

Hundreds of people demonstrated in Dalea on Saturday against earlier arrests, with some carrying the flag of the former South Yemen, which united with the North in 1990, residents and pro-southern websites said.

"The chief of security in Dalea said security forces were able to prevent and disperse a number of "elements" attempting to cause chaos, riot, and who were chanting slogans aiming at spreading hatred and causing division," the defence ministry said in its online newspaper on Sunday.

Yemen struck a truce on February 11 with rebels who have been fighting over religious, economic, and social grievances in the north.

On Saturday it imposed tougher security measures in Dalea, where separatist tensions have been rising. These included a ban on carrying weapons in public, two days after a policeman was shot dead in an ambush in a nearby province.

The policeman's death on Thursday brought to four the number of people killed in attacks on security personnel in the south in a week as authorities mounted arrest sweeps targetting separatists.

People in the south, home to most Yemeni oil facilities, complain that northerners have abused the 1990 agreement uniting the country to grab resources and discriminate against them. – Reuters