Protests spread in Bolivia on Monday over a violent crackdown on Indian marchers demonstrating against a highway planned to be built through an Amazon rainforest preserve.

A cabinet minister resigned in protest, students marched in major cities, and the Organization of American States (OAS) weighed sending a monitoring team to look into Sunday's clashes in a major crisis for the leftist government of President Evo Morales.

Riot police had fired tear gas to disperse a march on La Paz by Amazon Indians opposed to government plans for the highway. Police rounded up hundreds of 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."

Protesters reacted Monday by setting barricades ablaze on the airport runway in the town of Rurrenabaque in an attempt to free some 300 marchers, Mayor Yerko Nunez told local media.

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 planned road would run through a nature preserve home to some 50,000 natives from three Amazon Indian groups.

After more than a month of hiking, the protesters arrived just 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.

Morales, the country's first elected indigenous president, says the 300-kilometer (186 mile) highway is vital for economic development.

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, and a key outlet for Bolivian exports.

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

Amazon natives also fear landless Andean Quechua and Aymara people -- Bolivia's main indigenous groups and Morals 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 killed, or that people who fled the tear gas barrage are missing.

Llorenti insisted the police action was legal, but added that any excessive force from police will be sanctioned.

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 majority Aymara and Quechua Indians began a hunger strike -- as well as the northern province of Beni, a bastion of anti-Morales sentiment. Sixteen Amazon Indians also began a hunger strike in Santa Cruz.

At 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.