BOGOTA, COLOMBIA, FEB. 16 -- A powerful remote-control car bomb detonated outside a crowded bull-fighting arena in Medellin, killing 19 people, including six policemen, and wounding 60 others, authorities said tonight.

No one took responsibility for the attack, which police said was carried out with about 330 pounds of dynamite and was apparently aimed at a passing police patrol. Television showed about 35 vehicles destroyed in front of the Plaza Macarena bull ring, many in flames, severely hampering rescue operations.

Members of the Medellin cocaine cartel have taken responsibility for setting of dozens of similar car bombs in the past, but they have not used the tactic since July, and in a statement released Friday, the traffickers reiterated they were declaring a truce with the government.

Earlier today, Juan David Ochoa, a top Colombian drug lord accused by U.S. and Colombian authorities of being a leading member of the Medellin cartel, surrendered to justice officials in the northwestern town of Turbo, officials said.

Ochoa, on the run for years, is wanted for extradition to the United States to face drug-trafficking charges. He is the third member of the Ochoa family to accept President Cesar Gaviria's offer to surrender and confess in exchange for immunity from extradition and a shorter jail sentence.

While drug-related violence has captured most of the international attention, hard-line Marxist guerrilla groups have also been carrying out a major new offensive against the government, including an unprecedented wave of attacks on oil pipelines, causing almost 40,000 barrels of crude oil to spill into the nation's fragile ecosystem, according to government officials and ecologists.

The attacks are part of a campaign of economic sabotage by the National Liberation Army (ELN), a hard-line Marxist group whose main platform is to drive multinational oil companies from Colombia. A second guerrilla group, the Revolutionary Armed Forces of Colombia (FARC), recently has adopted the same tactic, denounced as "ecological terrorism" by the government.

Such attacks are not new, but they have increased dramatically this year. Ecopetrol, the state oil company, estimates that 650,000 barrels have been spilled since 1986, when the tactic was first used.

"In some areas, these attacks have affected crops and contaminated rivers where fishing is the main way of life," Gaviria said in a nationally televised speech last Monday, announcing new measures to combat the insurgents. "In the jungles and waterways, these terrorist attacks have spilled more petroleum in recent years than the {Exxon} Valdez spill in Alaska," measured at 260,000 barrels.

Gaviria also announced his government was willing to hold talks with the FARC and ELN to end the violence. According to the military press office, since Jan. 1, 328 people have been killed, including 142 guerrillas, 114 soldiers and 72 civilians.

A cousin of the president, Fortunato Gaviria, a former provincial governor who was believed kidnapped three days ago, was found shot dead on his farm today, police sources said. No one has claimed responsibility for the killing, the latest of several aimed at prominent public figures.

Ecopetrol, in figures released this week, said that in the first 40 days of the year, the guerrillas carried out 35 attacks against oil pipelines, spilling 38,000 barrels of oil. In all of 1990, the guerrillas carried out 23 attacks, spilling 100,030 barrels, it said.

The attacks are usually carried out by blowing up the pipe with dynamite, or by puncturing a section with gunfire and then setting it on fire.

Ecologists said the spills, which often occur in sparsely inhabited areas, are fouling the nation's rivers and lakes, killing aquatic life, poisoning the food chain and contaminating ground water.

"The ecological damage is irreparable," said Albert Ronald, a Colombian ecologist who is the Latin American director for Friends of the Earth, one of the few ecological groups operating in Colombia. "We do not have the technology to clean things up."

According to the Ecopetrol figures, in addition to the oil spilled, 2 million barrels of oil could not be pumped because of the pipeline damage. It estimated the total losses in revenue and repair costs at $86.5 million.