BOGOTA, COLOMBIA, APRIL 26 -- A gunman today pulled out an automatic pistol aboard a crowded Avianca jetliner and assassinated leftist presidential candidate Carlos Pizarro Leon Gomez before being killed by the slain politician's bodyguards.

Pizarro, 38, a former guerrilla commander who negotiated a peace agreement with the government in March and became an outspoken opponent of armed revolution, is the third presidential candidate to be killed in Colombia since August.

The shooting terrorized passengers aboard the Boeing 727, which had taken off from Bogota minutes before on a domestic flight. It returned quickly to the capital's airport.

An anonymous caller to a Colombian radio network said The Extraditables, the name used by the Medellin cocaine cartel, were responsible. The claim could not be verified, but authorities said the style of the killing and Pizarro's leftist stance pointed to the strongly anti-communist cartel, which killed the other two candidates.

Pizarro had abandoned armed insurrection along with 800 combatants of the M-19 guerrilla movement on March 8. The killing sent a shudder through other political campaigns because all major candidates have been threatened by the cartel. The prime target is front-runner Cesar Gaviria of the ruling Liberal Party, because of his strong anti-drug stand.

Elections are scheduled for May 27, but the M-19 and other leftist parties immediately asked that the voting be postponed because of insufficient security guarantees.

On Aug. 18, gunmen linked to the Medellin cartel killed leading Liberal candidate Luis Carlos Galan and on March 22 they gunned down leftist presidential candidate Bernardo Jaramillo, leader of the Patriotic Union, as he entered Bogota's airport.

Following the Jaramillo killing, security at airports was tightened. Passengers on domestic flights must arrive 90 minutes before the scheduled takeoff for searches. It was not clear how the gunman smuggled his weapon aboard.

Pizarro was flying on a commercial Avianca flight with 120 passengers from Bogota to the Caribbean coastal city of Barranquilla as part of his presidential campaign. Police said seven minutes after taking off, the gunman went to the restroom and came out with a mini-Ingram automatic pistol, approached the candidate from behind and emptied his clip of 15 bullets at point-blank range into his face, neck and hands. Some bullets hit the fuselage.

The gunman, identified as Alvaro Rodriguez, 25, was killed by Pizarro's bodyguards, who were sitting beside and behind the candidate. The plane immediately returned to Bogota and Pizarro was rushed to a nearby hospital, where he died at 11 a.m.

Police said the gunman was searched twice before boarding the flight, and he may have picked up his weapon in the bathroom of the airplane. A pilot of the airplane said, however, that the assassin wore the pistol strapped in his crotch, preventing detection.

An angry crowd of mourners gathered in front of the hospital, and when the death was announced, several pulled out pistols and fired into the air, causing panic. But M-19 leader Emiliano Sanchez said the group "will not fall into the trap of generating more violence, which is what these assassins want."

The military immediately went on maximum alert, suspended the sale of alcohol in the capital and closed schools and universities.

"Those who seek to submerge Colombia in barbarism will not prevail," President Virgilio Barco said on hearing the news. "This is a monstrous crime aimed at destroying our democracy and the process of reconciliation."

Pizarro's father was a vice admiral in the Colombian navy and served as naval attache in Washington. Pizarro joined the M-19 guerrilla movement when he was 19, and rose to command it. He was arrested in 1980 and spent three years in prison, then was freed under an amnesty.

The M-19 stands for the April 19 Movement, which carried out a series of spectacular and bloody attacks across the country in the 1980s, including the Nov. 26, 1986, takeover of the Palace of Justice, where more than 100 people, including Colombia's chief justice and 11 of the 24 associate justices were killed.

Pizarro exchanged his fatigues and boots for suits and ties during the campaign and spoke often of the personal risks of his campaign. He was the only major candidate to continue campaigning in public plazas.

"We cannot allow death to keep us from doing what must be done for the good of democracy and the country," he told his troops the day they turned over their weapons. "Our lives are at risk, and we know it is tremendously difficult to act for the good of our country."

Pizarro ran for mayor of Bogota in the March 11 municipal elections and, despite having only two days to campaign, finished third, ahead of more established leftists.

After the killing of Jaramillo in March, Pizarro helped found an alliance of 12 leftist parties and was chosen its presidential candidate. He was strongly nationalistic and opposed extradition of drug traffickers to the United States.

The latest polls showed him with about 5 percent of the vote, small but significant in a country where the legal left has been the victim of a campaign of extermination by the far right, linked to drug traffickers.

Pizarro's campaign theme was peace, and his TV spots showed him wrapping his pistol in the flag and putting it on a pile of surrendered weapons. He had grown critical of Marxist Cuba and Nicaragua's Sandinistas.