Residents said hundreds of soldiers lined the streets of Aden, where southern activists had been planning a festival to commemorate the day the last British soldier departed in 1967.
In the run-up to the anniversary several clashes erupted between the Sanaa government and southerners, who have long complained that northerners abused a 1990 unity agreement to exploit their resources and discriminate against them.
Southern activist websites said security forces had blocked off all entrances to Aden, where the authorities had told people not to hold gatherings or demonstrations without a permit.
Yemen’s government is already fighting a revolt in the north by minority Zaidi Shi’ites, who also complain of neglect and oppression. Neighbouring Saudi Arabia was recently drawn into the northern conflict when rebels seized some Saudi territory.
The rebels said on their website (almenpar.com) that Saudi Arabia renewed its air and missile attacks on several locations near the border early yesterday. They accused Saudi forces of using phosphorus bombs and poison gas in the attacks.
There was no immediate reaction from Saudi Arabia, the world’s largest oil exporter, which fears growing instability in Yemen is giving al Qaeda an opportunity to strengthen its foothold there.
Last Sunday, activists shot dead a soldier in the southern province of Shabwa. A second soldier died of wounds inflicted in fighting there last Wednesday, a security official said.
Secessionists also clashed with the armed forces in the Radfan region last Sunday, when one person was killed and a grenade hit the local intelligence headquarters, the same official said.
Members of the activist Southern Movement killed three northerners in two carjackings, the state-run website September 26 (26sep.net) said. But the pro-southern website adenpress.com quoted Nasser al-Khabji, a leader of the movement, as denying its members were involved in the incidents.
Violence erupted this year after an April 28 opposition rally to mark the 1994 civil war in which President Ali Abdullah Saleh’s forces defeated the secessionist south, known before the unity deal as the People’s Democratic Republic of Yemen.
Protests led by army officers, riled by their meagre pensions after forced retirement, turned violent in 2007.
Southern leaders have talked of northern "occupation" and called for secession amid mounting discontent over jobs and other economic grievances.