CROSSMAGLEN, Northern Ireland — Irish Republican Army supporters armed with clubs, fence posts and other makeshift weapons clashed with riot police and soldiers Sunday at three British security installations near the Northern Ireland border.

Police said 22 officers suffered mostly minor injuries, but two with head wounds were airlifted by helicopter to a Belfast hospital. The officers reported firing two plastic bullets to force back protesters outside one army watchtower and used batons and another plastic bullet to beat back protesters who had forced their way into the main barracks used by soldiers and police in Crossmaglen.

One soldier suffered burn injuries after protesters filled a large oil drum with gasoline and set it alight against the barracks' main gate, police said. Two military dogs were also injured in the attack on the base.

Police said they had arrested four people during Sunday's protests — two men and two teenagers — and had videotaped the scuffles outside the watchtowers in the hope of identifying people in the crowd. Warning signs outside each base read, "This is a prohibited place within the meaning of the Official Secrets Act. Unauthorized persons in the area will be arrested and prosecuted."

The protests, organized by the youth wing of the IRA-linked Sinn Fein party, were designed to vent local opposition to the continued deployment of British troops in South Armagh, an overwhelmingly Catholic region of the Northern Ireland border where anti-British sentiments run high.

The protesters, some of them wearing masks and carrying Gaelic hockey sticks, gathered first at Crievekieran watchtower, an installation that allows soldiers to monitor traffic and conversations for several miles around.

They began pulling up fence posts and digging with shovels before being confronted by about a dozen police equipped with helmets, shields and clubs.