AFP - Yemeni police banned southern separatists from staging a demonstration marking Britain's 1967 withdrawal from the Arabian Peninsula country after gunmen from the south killed two northerners on Monday.

The armed men ambushed one northerner at a roadblock near Radfan, about 360 kilometres (225 miles) south of the capital, Sanaa, killing him and seizing his car, said Jasser al-Yamani, deputy governor of southern Lehej province.

They then stopped another man who was returning to Sanaa after spending the Muslim holiday of Eid al-Adha in the main southern port city of Aden, shooting him dead in front of his family.

The murders were followed by a crackdown in which more than 200 people were arrested in Aden, and blockades were also put up around the city to prevent others from attending the demonstration, according to security services.

But former Yemen vice president Ali Salem al-Bidh, a leading southern separatist, accused the security forces of "arresting more than 2,000" demonstrators and "opening fire on civilians."

Bidh, who lives in exile in Germany, said in a statement received by AFP that his people were "conducting a peaceful struggle to end the occupation" and called on Sanaa to "withdraw its troops quickly and unconditionally."

He appealed for the United Nations, Arab League and the Organisation of the Islamic Conference "to establish a mechanism to protect our people in accordance with the Geneva Conventions."

Secessionists had been planning to stage a rally in Aden to mark the anniversary of the British pullout that heralded the independence of the former South Yemen on November 30, 1967.

Five people, including two soldiers, were killed in clashes at a similar demonstration on Wednesday.

Known from 1970 as the People's Democratic Republic of Yemen and run by a socialist government, the south was an independent nation from the 1967 British departure until 1990.

The current unrest has its roots in the years after unification, particularly following the 1994 civil war.

That conflict began when Bidh proclaimed the secession of the south in May that year, and it ended after northern troops invaded the region.