A huge Yemeni police presence blocked southern protesters from entering the region's main city Aden on Monday ahead of the launch of the Gulf Cup football tournament.
President Ali Abdullah Saleh was joined at the opening ceremony by his visiting Ethiopian and Djiboutian counterparts.
Police were deployed on all the city's main thoroughfares as well as around the stadium and team hotels ahead of the opening game between the hosts and oil-rich neighbor Saudi Arabia, AFP correspondents said.
Troops blocked hundreds of supporters of the Southern Movement, a coalition of autonomist and pro-independence groups, from entering the city from neighboring Daleh province, a major centre for protests in recent months, security sources said.
Southern Movement sources said that troops posted in the town of Anad, some 65 kilometers (40 miles) north of Aden, opened fire on demonstrators carrying flags of the formerly independent south.
They also carried pictures of the leaders of the seven countries competing against Yemen in the two-week tournament -- Bahrain, Iraq, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates.
The Southern Movement had called on its supporters to demonstrate during the event, which continues until December 5.
The authorities have mobilized some 30,000 police to provide security for the visiting teams and fans, the tournament's security chief General Saleh Zawara said on Sunday.
It is the first time that the Arab world's poorest country has staged the tournament since its launch in 1970.
The authorities have been at pains to reassure participating countries that the security forces are able to deal with the threat of disruption by the Southern Movement as well as a growing al-Qaeda presence in the region.
Several rings of checkpoints were set up around the May 22 Stadium in the southern port of Aden, the host city for the eight-nation event, and police earlier stopped a protest rally called by southern separatists.
A capacity crowd of more than 30,000 spectators was expected for Monday's opening matches between Yemen and Saudi Arabia and, later, Kuwait and Qatar.
The event's opening ceremony featured an artistic display by about 1,500 dancers from Yemen and other Gulf countries that reflected the region's history and cultural life.