Farmers, doctors and politicians, some dressed as ghosts and skeletons, marched through central Lima to decry a U.S free-trade deal they see as a way of bullying Peru to allow U.S. companies to dominate its markets.

"We're really worried about the impact this is going to have. Not only are medicines going to get more expensive, but fewer people will have access to drugs," said Manuel Izaca, president of the association of Peruvian drug companies.

Colombia, Ecuador and Peru are negotiating the accord with the United States aimed at boosting trade flows and economic growth in the Andean nations. The Andeans had hoped to strike a deal at the start of this year. Peru now hopes to conclude in September.

In Geneva, U.N investigator Paul Hunt of New Zealand urged the United States not to pressure Peru into a trade pact and suggested the deal would oblige Peru to enforce tougher patent protection rules than those of the World Trade Organization .

To meet its international human rights responsibilities, he said: "The U.S. must not apply any pressure on Peru to enter into commitments that are inconsistent with Peru's constitutional and international human rights obligations."

Hunt, an independent expert who monitors how U.N. accords on the right to health are observed around the world, said he was concerned that the accord would water down health standards and bring "higher prices for essential drugs that millions of Peruvians would find unaffordable."

Jose Carlos Vera, an economist specializing in the health sector, told Reuters he estimated a free-trade deal would increase the cost of medicines in Peru by $34 million in the first year, a figure that could grow to $168 million in 2017.

The U.N. says that some 50 percent of Peru's 28 million people live in poverty and that many die from treatable illnesses because they cannot obtain, or pay for, drugs

But a U.S. official said the deal could actually bring down drug costs because it would foster competition and iron out distortions in Peru's internal market.

"You see it already with toothpaste. A new competitor comes onto the market and prices begin to fall," said Timothy Stater, Economic Councilor at the U.S. embassy in Lima.

Peruvians' fears echo concern among U.N. and non-governmental groups that Washington, under pressure from pharmaceutical firms, may be pushing for tough patent terms in bilateral trade pacts with poorer nations.

Stater said the United States wants to negotiate a 5-year data protection period on new medicines, while Peru wants the duration to be only three years.

"Five years is not a long time, it's less than with other trade accords and it's in line with the World Trade Organization norms," he said.

The United States and Peru, Colombia and Ecuador will hold a fresh round of the trade pact talks in Miami on July 18-22.