Colombia's largest cities were braced on Thursday for marches by students and labour unions in support of a growing nationwide strike by miners, truckers, coffee growers, milk producers and potato farmers protesting against everything from high fuel prices to free trade agreements that farmers say have them on the brink of bankruptcy.

The protests began on 19 August, with demonstrators joining striking miners,to block some of the country's main highways using tree branches, rocks and burning tyres. At least one protester and one policeman have died in the demonstrations, dozens have been injured and more than 150 have been arrested.

The protests spread to the cities where residents banged pots in solidarity with the farmers after president Juan Manuel Santos, in a failed effort to downplay the importance of the strikes, said the "supposed national farmers' strike does not exist".

Forced to apologise for the statement, he sent out high-level officials to begin negotiating separately with the different sectors. "We recognise that the farmers' protests respond to real needs and problems. We are listening to them and offering solutions," Santos said on Wednesday night.

Farmers complain that agricultural imports allowed under free trade agreements with the US, the EU, Canada and other nations are undercutting their livelihoods.

Strike leaders said solutions offered – eliminating import tariffs on fertilisers, easing agricultural credit and triggering protective safeguards allowed under the free trade agreement for sensitive sectors – were not enough. "Talks continue," strike leaders said on Wednesday, but called on farmers to continue protests.

Thursday's marches are planned for Bogotá, Medellín, Cali and other mid-sized cities. With access to cities from the countryside disrupted, the price of some staple foods has nearly doubled. Gloria Galindo, a mother of three who lives in Bosa, a working-class district of Bogotá, said she sympathised with the protesters but that the roadblocks were hurting her family's wallet. "Vegetable prices have shot through the roof," she said.

Officials have accused leftist guerrillas of the Revolutionary Armed Forces of Colombia (FARC), which is in talks with the government to end nearly 50 years of war, of infiltrating the strikes. Strike leaders have denied the claims.

Rural development was the first point of agreement between the Farc and the government in the peace process. But the current protests show that Colombia's conflicts are not limited to an armed insurgency and will not necessarily be resolved at the negotiating table in Havana, according to Alejandro Reyes, an adviser to the government on land issues. "When we get to a post-conflict stage there will be an enormous social conflict to deal with," he said.