MEXICO CITY (AP) - Anti-globalization activists predict that millions of Mexican farmers will stream into the United States when Mexico lifts tariffs on U.S. farm products in January.

The government says the import opening will make Mexican farms more competitive, bring new investment to the countryside and give factory jobs to those who now eke out a living on antiquated, overpopulated farms.

President Vicente Fox has come under pressure to rip up free trade accords and spend more subsidy money to protect Mexican farms. Fox, a free trade supporter, has begun to concede that the opening could create problems.

"I want to talk to President Bush about the subsidies that the American government has decided to give and how to prevent them from affecting Mexican communities and producers," Fox told the Associated Press in a recent interview. "Otherwise, what you'll have is more migration."

For Americans already affected by two decades of large-scale illegal immigration, the dire warnings may seem a bit late.

Farm activist Luis Hernandez says things will get even worse in January, when the North American Free Trade Agreement eliminates tariffs between the United States and Mexico on all but three products: corn, dairy and sugar. All tariffs will be removed in 2008.

"For the Mexican countryside, this is going to be devastating," Hernandez said. He predicted "an increase in migration, in bankruptcies, violence and drug growing" as Mexican farmers turn to the only crops that can be profitable for them.

Farmers have already slaughtered cows and dumped pineapples on Mexico City streets, seized highways and blocked bridges to defend a way of life on the hardscrabble farms where corn was first domesticated 4,000 years ago.

Today, Hernandez said, those farms largely serve as "vast parking lots for the unemployed." And they are full: while urban Mexican families have an average of 2.4 children, women in poor rural communities continue to have an average of four to five.

That growth - coupled with gradually disappearing farm jobs - is the main engine for the exodus. In 1990, Mexico had 9.8 million farmers; in 2000, there were 8.6 million.

Many go to Mexico's burgeoning cities. Others leave for the United States, and migration could swell from 300,000 a year to 500,000 a year by 2030, according to Mexico's National Population Council.

The farms migrants leave behind are tiny. Seventy percent of farmers have fewer than 12 acres. Many are communal plots that lack clear titles, making it hard for farmers to modernize, get loans or combine smaller plots into viable farms.

As a result, many don't live off their crops and haven't for years.

"A lot of people who call themselves farmers really aren't," said Ricardo Celma, Mexico representative of the U.S. Grains Council. "They're taxi drivers with some land."

Mexico does have highly productive commercial farms, especially in the north of the country, but they pay laborers as little as $2 a day and sometimes employ children.

With farmers in such straits, NAFTA is an easy target - even though most of the tariffs to be removed in 2003 are already as low as 2 percent.

Some people are threatening social unrest unless NAFTA is overturned. The radical farm group El Barzon is among those that say NAFTA creates unfair competition and could cause widespread poverty that will result in acts of desperation.

Yet, while U.S. subsidies are an easy target, they may not be the real problem. Mexico's corn subsidies average $150 a ton, well above the $85 that U.S. farmers are paid.

The problem is that most Mexican farmers produce so little they cannot get by even with subsidies. Mexico's 8.6 million farmers produce about one-seventh as much as their 3 million U.S. counterparts.

"No country in the world could provide jobs for this many people in the farm sector," said Humberto Jasso, of Mexico's Economy Secretariat. "We have to provide nonfarm jobs in rural areas, and that's an area where NAFTA can help."

But farm supporters have joined anti-globalization activists in opposing plans to bring factory jobs to rural southern Mexico, saying the work would destroy Indian cultures rooted in the farming life.

"We have to stop seeing farms as just mercantile production," said Victor Suarez, director of the leftist farm group ANEC. "Instead, we have to recognize people's right to be farmers . . . and pay them for conserving their land, their culture."

But others point out that Mexico already spends $8.7 billion a year on direct and indirect farm subsidies, close to the Federal Education Department's $10.4 billion budget.

"The only thing subsidies would do is sink the whole country into poverty," said political analyst Sergio Sarmiento. "What we have to do is modernize land holdings and do away with communal farms."