At least four people were arrested and 12 police officers injured in the melee in downtown Bogota, the city police chief, Gen. Luis Eduardo Martinez, said. Red Cross officials said 10 civilians were hurt.

None of the injuries appeared serious in what was Bogota’s worst street violence since protests in March 2012 against the city’s troubled municipal bus system were similarly blighted by young vandals.

Thursday’s mayhem lasted about four hours and some of the rioters threw small homemade explosives known as “potato bombs” at police.

The clashes occurred just hours after President Juan Manuel Santos acknowledged in a nationwide TV address that “a storm” is battering Colombia’s agricultural sector and promised remedies.

He opened talks with the protesting farmers Tuesday.

Santos said the estimated 45,000 farmers, coffee growers and truck drivers who have blocked highways and battled with riot police since last week had justified complaints resulting from years of government neglect. He announced measures to address the troubles including removing import tariffs on 23 types of fertilizers. He did not say when they would take effect.

The farmers have demanded lower fertilizer prices. They complain of being undercut by cheap imports from near and far of products including potatoes, onions and milk. They say their sector is being hurt by free trade and other agreements promoted by the government.

At least two people have been killed in the protests and at least 175 arrested, according to human rights groups, which have complained of police brutality. The government has reported at least 72 road blockades going up and being removed across the country during the strike, with at least 29 vehicles destroyed.

On Thursday, however, relative calm was reported in the countryside.

Santos, an economist and former defense and foreign trade minister, has been buffeted by protests since taking office in August 2010.

University students took to the streets the following year to demand reforms. Truck drivers have protested high gasoline prices, while indigenous groups have demanded security forces and guerrillas quit their territory. This year, coca-growers have blocked highways in a turbulent northeastern region to demand an end to aerial spraying with herbicides.

While analysts don’t believe the unrest poses a serious threat to Santos’ government, they say it could certainly cost him votes in the May 2014 presidential election.

Santos has not yet announced whether he will seek re-election, and the far bigger issue for voters is his management of peace talks with Colombia’s largest rebel group to end a half-century conflict that has claimed more than 200,000 lives.

Political scientist Sandra Borda of the Universidad de los Andes said Santos may have failed to anticipate the unrest but it cannot be compared with the protests that recently rocked Brazil.

“While in Brazil it was the middle class, these protests originate in the farming sector, which does generate solidarity (from other) sectors who feel neglected,” she said. Those include university students, some of whom marched Thursday in the woolen ponchos worn by highlands farmers.

The students are demanding the government pay off a $5.7 billion higher education deficit that has accumulated over more than two decades.

For safety reasons, Bogota’s city government suspended classes Thursday for public school students.

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Associated Press writers Vivian Sequera in Bogota and Frank Bajak in Lima, Peru, contributed to this report.