By Jeremy McDermott BBC correspondent in Antioquia, Colombia

Colombia's right-wing paramilitaries have invaded a town in rebel territory in the northern province of Antioquia.

The operation was part of a strike back by the groups which have been suffering under an offensive by Marxist guerrillas.

Hundreds of people have been displaced and there are fears over the fate of 120 people who are believed to be in paramilitary hands.

The town of Yondo - on the banks of the Magdalena river - had been under the control of guerrillas of the Revolutionary Armed Forces of Colombia (FARC) who were regularly seen passing through this strategic area.

President Alvaro Uribe has promised democracy and security

But boats carrying 400 heavily armed paramilitaries landed in the town, prompting Colombia's human rights watchdog, people's defender Eduardo Cifuentes, to call an emergency press conference to highlight the plight of the Yondo community.

Hundreds of civilians have fled the paramilitary invasion and more than 100 people are unaccounted for.

The paramilitaries have traditionally used a policy of massacre.

They arrive in communities in rebel territory and execute suspected guerrilla sympathisers to terrorise the locals into ending support for the insurgents.

Crossfire dangers

There is also a fear that civilians could be killed in the crossfire, should the FARC react to this paramilitary invasion.

Some 109 civilians were killed in the town of Bojaya on the other side of Antioquia in May, when paramilitaries occupied the town and guerrillas bombed it, killing the inhabitants who had taken shelter in the town church.

The security forces are nowhere to be seen in Yondo - a reminder for the new hard-line President Alvaro Uribe that he still has much to do before he can claim to have implemented his campaign promise of democratic authority and basic security for the Colombians who live in the half of the country under the control of the warring factions.