At least 3,000 coca growers have marched on the Peruvian capital, Lima, to protest at government plans to eradicate their crop.
Growers say protests will continue if their demands are not met
The growers, from Peru's coca-growing regions, demanded an end to the destruction of coca crops - the raw material used to make the drug cocaine.
They also demanded freedom for their leader, in prison on terrorism charges.
Although some coca use is legal, Peru's anti-drugs agency says 83% of production is turned into cocaine.
The growers say coca leaves, which are chewed or used in tea, play an important role in Peruvian culture.
But the government says the only people to benefit from the protest will be drug traffickers in Peru, which is the world's second-largest cocaine producer behind neighbouring Colombia.
Demonstrators chanted slogans such as "we are farmers, not criminals" and waved flags and banners painted with the names of their regions.
One of the organisers, Elsa Malpartida, said the government was damaging the environment as well as farmers' livelihoods.
"They've put lots of poison in our fields to kill the coca, and the poison that they are using is not only killing the coca, it's killing all types of products," she told the BBC.
Many protesters collapsed with fatigue after their 11-day journey
"It's also causing cancer which is affecting the farmers there."
The farmers say if the government does not listen to their demands, they will block roads and stage a hunger strike.
Many of the protesters collapsed on the ground under a baking sun after their 11-day journey by foot and by bus, which began hundreds of miles away in the Amazon region, says the BBC's Hannah Hennessy in Lima.
Hundreds of police officers carrying gas masks, tear gas and rifles accompanied the protesters as they arrived at the Peruvian Congress, where representatives held a short meeting with politicians.
There were scuffles early on Tuesday morning when police evicted demonstrators from their makeshift camp outside the Palace of Justice. One protester was arrested.