Gunmen killed a policeman in Yemen's main southern city of Aden, where separatists were briefly arrested Wednesday ahead of a planned demonstration to demand independence, police said.
"Unknown assailants on Wednesday ambushed a police patrol in Mualla (an Aden district)," a security official said. They "hurled a homemade bomb at it and opened fire, killing a policeman."
The same official source said police "freed 17 Southern Movement activists, hours after they were arrested at Al-Arood Square" in Aden, where they were preparing for the rally.
The situation was tense in Aden, where security forces were heavily deployed and have set up roadblocks to prevent the rally called for by hardline factions of the Southern Movement.
Factions of the Southern Movement demanding complete secession of the formerly independent south, have boycotted national reconciliation talks that began in Sanaa on March 18 and which have so far resulted in an initial agreement to turn Yemen into a federal state.
Southern delegates have been demanding a federal state consisting of north and south Yemen, while northerners are proposing more than two regions.
After the former North and South Yemen united in 1990, the south broke away in 1994, triggering a brief civil war that ended with the region being overrun by northern troops.
Hassan Baoum, head of the Southern Movement's supreme council, has called for the protest, still expected to take place later on Wednesday.
A police official told AFP local authorities in the south were determined to "prevent anyone from causing chaos in Aden."
The UN-backed dialogue aims to draw up a new constitution and prepare for elections in February. It had been scheduled to end on Wednesday but was extended by a month over discussions on the shape of the future federal state.