Mexico City (AFP) - Mexico's presidential candidates sparred feistily on the crime and corruption racking the country in their first debate Sunday, but achieved near-unanimity on one issue: attacking the leftist front-runner, Andres Manuel Lopez Obrador.

Recent opinion polls give Lopez Obrador a double-digit lead heading into the July 1 election, putting the fiery former Mexico City mayor squarely in the sights of his four rivals as they held their first of three debates.

The other candidates launched a relentless series of personal and policy attacks, especially tearing into Lopez Obrador's flirtation with one controversial proposal: granting an amnesty for drug crimes in an effort to halt record-breaking violence linked to narcotics trafficking.

Hit on all sides, Lopez Obrador -- widely known as "AMLO" -- at one point whipped out a chart with the latest poll numbers by way of rebuttal.

"I don't want to brag, but I humbly submit to you the latest poll: 48 percent support," he said in a mocking tone.

In Mexico's first-past-the-post election system, that would be enough for him to win in his third bid at the presidency, given that the remainder is split between the rest of the crowded field.

It remains to be seen whether the debate will change those numbers.

Second-place candidate Ricardo Anaya, who is running for a coalition between his conservative National Action Party (PAN) and two leftist parties, called the amnesty proposal "a crazy idea that would generate enormous violence."

When Lopez Obrador, 64, said he was not proposing "impunity" but simply a national dialogue on the amnesty idea -- with a panel of experts including Pope Francis -- Anaya laid into the front-runner for waffling.

"He tricks people. He tailors his message to whoever his audience is, and since today everyone is watching, he doesn't know what to say," he said.

Anaya, 39, had 26 percent support in the Reforma newspaper poll cited by Lopez Obrador.

A baby-faced political whizz who was formerly speaker of Congress, his rapid rise has been marred by corruption allegations that he denounces as spurious.

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Third-place candidate Jose Antonio Meade of the ruling Institutional Revolutionary Party (PRI), who had 18 percent in the Reforma poll, joined the attacks on Lopez Obrador, saying his "ambition for power" had turned him into the "puppet of criminals."

But Meade, a respected former finance and foreign minister, also had to fend off fierce criticism of the ruling party and deeply unpopular President Enrique Pena Nieto.

- Game changer? -

Two independent candidates with single-digit poll numbers, former first lady Margarita Zavala and Nuevo Leon state governor Jaime "El Bronco" Rodriguez, meanwhile sought momentum.

Zavala defended her husband, ex-president Felipe Calderon of the PAN, who has been criticized for deploying the army to fight Mexico's powerful drug cartels -- often called a catalyst for the current wave of violent crime.

But she said she would not be beholden to her husband's policies.

"I will eliminate this violence with values," she said.

Rodriguez, a tough-talking former rancher who does not shy away from controversy, for his part proposed amputating the hands of corrupt officials.

"Literally?" a surprised moderator asked him.

"Yes," he said. "We have to cut a hand off the criminals."

The topics for the debate were corruption, public safety and violence.

Lopez Obrador largely played it safe, repeating well-worn stump speech lines.

Often criticized by enemies as a hot-headed tyrant, he pointedly kept his cool as his rivals tried to provoke him.

Political analysts were split on whether the debate would make a difference in the race.

It could be an "inflexion point" for Anaya, said noted commentator Leo Zuckermann.

"He managed to score some major blows on Lopez Obrador," he told Televisa TV.

"Meade loses, because it was now or never for him, and Anaya beat him in looking like the candidate capable of defeating Lopez Obrador."

Historian Hector Aguilar Camin disagreed.

"Anaya won the debate, but it won't make any difference," he said.