Thousands of Colombian farmers and state workers have marched through Bogota.

They banged pots and pans after 11 days of increasingly violent protests against agricultural and trade policies they say have left them impoverished.

Students wearing balaclavas pelted shop windows with rocks near the capital's main square and clashed with riot police who fired tear gas to disperse them.

The government imposed a curfew in three of the more populated areas of the city after violence continued into the night.

"Long live the farmers' strike! Food sovereignty," protesters chanted as they waved anti-government banners.

President Juan Manuel Santos has been unable to end the so-called national strike that has united potato growers, milk producers and teachers.

He has acknowledged agriculture is in crisis, but called for peaceful dissent while talks about possible solutions are going on.

"The farm sector has been abandoned," the centre-right president said in a televised address yesterday.

"The protests are valid ... but, via dialogue, we will resolve the problems ... We are in a storm, but we will persevere."

Protesters wearing typical farmer attire of woolen ponchos, brimmed hats and rubber boots to show their solidarity, marched in 15 columns toward the Plaza Bolivar, where the presidential palace and Congress are located.

There were also protests in Medellin, Cali and elsewhere across the country.

Farmers blocked roads, snarling city-bound traffic and piling pressure on Mr Santos three months before he must decide whether to run for a second term.