Anger grew in Bolivia on Monday over a violent crackdown on indigenous groups protesting the construction of a major highway through an Amazon rainforest preserve.

A cabinet minister resigned in protest, students marched in major cities, a national strike is being planned, and the Organization of American States (OAS) considered sending monitors to look into Sunday's clashes.

The protests amount to a major challenge to the leftist government of President Evo Morales, the country's first elected indigenous president, who says the 300-kilometer (186 mile) highway is vital for economic development.

Riot police fired tear gas Sunday to disperse a march on La Paz by hundreds of people opposed to the highway plan. Police rounded up some 300 marchers and forced them onto buses in an operation that left several people injured.

Police surged into the demonstrators' camp with "extreme violence," veteran rights activist Maria Carvajal told AFP. "I could not believe what was happening."

On Monday protesters blocked the airport runway in the town of Rurrenabaque with blazing tires and tree trunks to prevent police from flying the arrested marchers out, Mayor Yerko Nunez told local media.

Crowds of sympathetic residents later chased off the police and released the detainees, who at first fled into the countryside but later began to trickle back to town, Nunez said later.

"We don't understand why the government has acted in this brutal manner," said Rafael Quispe, one of the protest leaders. "This is a government that says it is of the indigenous people, yet it has attacked them."

Defense Minister Cecilia Chacon announced she was resigning in protest over the crackdown.

"I do not agree with the intervention in the march and I cannot justify the measure when other alternatives existed," she said in a letter to Morales.

The Brazil-financed road would run through a nature preserve home to some 50,000 natives from three different Amazonian indigenous groups.

After more than a month of hiking, protesters arrived outside the town of Yucumo on Saturday after breaking through a police barricade by forcing the government mediator, Foreign Minister David Choquehuanca, to march with them.

The road is part of a network linking land-locked Bolivia, South America's only mostly indigenous nation, to both the Pacific through Chile and the Atlantic through Brazil, a key outlet for Bolivian exports.

The government says it would be too costly to build the highway around the preserve.

Amazon natives also fear landless Andean Quechua and Aymara people -- Bolivia's main indigenous groups and Morales supporters -- will flood into the road area and colonize the region.

"The most important thing for us is that they stop the violence as soon as possible," said the UN's envoy in Bolivia, Yoriko Yasukawa, reminding authorities it was their responsibility to "protect the people."

Speaking at the Quemado government palace in La Paz, Interior Minister Sacha Llorenti denied reports that an infant traveling with the protesters had been killed, or that people who fled the tear gas barrage were missing.

Riot police set up a security cordon around the Quemado building as thousands of demonstrators outside, mainly college students, protested the crackdown. Violence, however, was limited to a few tomatoes hurled at policemen.

Other protests were held in the central city of Cochabamba -- where students marched and Aymara and Quechua groups began a hunger strike -- as well as in the northern province of Beni, a bastion of anti-Morales sentiment.

Pedro Montes, the leftist leader of the Central Obrera Boliviana (COB), the country's powerful labor federation, said that a national strike had been called for Wednesday to protest the weekend crackdown.

At the OAS headquarters in Washington, two senior officials insisted there was no need for human rights monitors to travel to Bolivia.

A cabinet minister and Hector Arce, the speaker of Bolivia's chamber of deputies, met with OAS rights officials and Secretary General Jose Miguel Insulza to discuss the weekend events.

In an attempt to defuse tensions, Morales said Sunday he would hold a referendum on whether to build the road. No date was set for the vote, and Morales made no further public appearances.