Tens of thousands of people have taken to the streets of the Colombian capital, Bogota, in support of a strike by small-scale farmers.
The farmers say the government's agricultural policies are driving them into bankruptcy.
The president said the demonstrations were valid, but urged protesters to keep them peaceful. Some marchers have clashed with police.
Negotiations to end the protests, which started 11 days ago, remain deadlocked.
The protests have united potato growers and milk producers with teachers, health workers and students.
They all converged on Bogota's main square, Plaza de Bolivar, on Thursday to make their grievances heard.
"Long live the farmers' strike," they chanted, holding up protest banners.
At Plaza de Bolivar some protesters wearing balaclavas clashed with riot police, who responded with tear gas.
Businesses closed to prevent looting.
Some 20 people have been injured and a number arrested.
Interior Minister Fernando Carrillo called people resorting to violence "vandals, not farmers."
"We are convinced that we will come to an agreement on our agricultural policy after the crisis is over," he said.
Protesters in other parts of the country have been blocking roads and disrupting food supplies to major cities and towns.
Dialogue is the only way
President Juan Manuel Santos said it was the right of farmers to demonstrate against his government.
But he called on protesters to reject violence.
"Please, let us not fall into the hands of the violent," he said."It would lead to unnecessary confrontations and deaths."
Mr Santos said the only way to find a solution was through dialogue.
On Wednesday the government announced measures - including better prices for agricultural products and more access to loans - to ease the pressure on farmers.
The government also promised more protection from products imported at lower prices from countries with free trade agreements with Colombia.
But the small-scale farmers so far have rejected the government's offer.
They say that free trade agreements with the European Union and the United States, which have recently come into force, are flooding the market with agricultural products at prices they are unable to match.
They also complain that rising fuel and production costs have turned small-scale farming into a loss-making business.