At least 12 people have been killed in clashes between armed separatists and government forces in southern Yemen, medics and witnesses say.
The protesters in the town of Zinjibar, in Abyan province, were demanding the release of detainees held during earlier disturbances.
Eyewitnesses said the security forces opened fire to disperse the crowds.
Many in southern Yemen complain of discrimination, while officials accuse the protesters of seeking secession.
Witnesses told the AFP news agency the demonstration was called by Tarek al-Fadhli, a local figure who supports independence for south Yemen from the north.
Analysts say there has been rising tension throughout the south in the past two years, as the southern independence movement gains strength.
It began two years ago when former southern military officials, forced into compulsory retirement, demanded higher pension payments.
The protesters have been accusing President Ali Abdullah Saleh of corruption and openly calling for independence from his government in the northern mountain capital, Sanaa.
Mr Fadhli - a prominent ally of President Saleh and a veteran of the jihad against the Soviet Union in Afghanistan - switched sides and joined the southern independence movement in April.