Updated

The issue of drug decriminalization split Mexican politics in strange ways on Saturday, after President Vicente Fox refused to sign a bill that would have eliminated criminal penalties for possession of small amounts of drugs.

About 500 protesters held a marijuana smoke-in in Mexico City, and a presidential candidate who visited the demonstration came out in favor of decriminalization. Mexico City's police chief came out against it, and some members of Congress accused Fox of yielding to U.S. pressure to veto the bill.

"Decriminalization does not create more users ... we have to decriminalize the discussion of decriminalization," said presidential candidate Patricia Mercado, of the small Alternative Social-Democratic party, during a visit to the smoke-in and protest at a park in downtown Mexico City, where youths openly smoked joints and a heavy odor of marijuana hung in the air.

Mercado declined protesters' invitation to "Light up! Light up!" but said she did support decriminalizing marijuana.

A half-dozen Mexico City police officers confronted the protesters, but the crowd thronged around them shouting "Take us all, Take us all!" and the police quickly retreated.

Possession of marijuana is currently a crime, punishable by 10 to 16 months in prison, unless a suspect can claim he is an addict or it is a first offense involving a small amount. However, few people are currently prosecuted under the law.

Protest organizers described comments by U.S. officials asking Mexico to reconsider the bill as a violation of Mexico's sovereignty.

"The president has declared war on (drug) consumers," said Alfonso Garcia, secretary of the Mexican Association for Cannabis Studies, who described the bill Fox sent back to Congress on Wednesday as "a minor advance."

But the police chief of Mexico's capital — like Mercado, a leftist — said Saturday he supported Fox's decision not to sign the bill into law.

Joel Ortega said it would have made it harder for his officers to fight violent drug gangs.

"Imagine for a moment that we are doing a raid, we'd almost have to say, 'Let's see, gentlemen drug traffickers, allow me to weigh the drugs to see if we have the power to arrest you,"' Ortega told a news conference.

Conversely, many legislators — including members of Fox's conservative National Action Party — supported the bill. They continued to defend it this week, and accused Fox of bowing to U.S. pressure.

"Unfortunately, very unfortunately, the president, under pressure from the United States, sent it back to Congress, saying it would 'regularize' drugs, which is not true," said Rep. Marcela Gonzalez Salas, of the leftist Democratic Revolution Party.

The measure would have dropped criminal penalties for possession of small amounts of marijuana, cocaine, heroin and other drugs. It also proposed rules on whether larger amounts of drugs could be seized by city police or federal agents, depending on the quantity.

Officials in Washington expressed concern that decriminalization would encourage "drug tourism" to Mexico and increase the amount of narcotics available in border cities. On Thursday, U.S. State Department spokesman Sean McCormack applauded Fox's decision not to sign it.

Under the proposed law, Mexicans would have legally been able to possess up to 25 milligrams of heroin, 5 grams of marijuana (about one-fifth of an ounce, or about four joints) or 0.5 grams of cocaine — the equivalent of about four "lines."