The Drug Enforcement Agency is under siege. Latin American cocaine lords, angered by recent U.S. successes in the war on illegal drugs, have declared war on the DEA.

As a result, the DEA has to divide its efforts between combating narcotics trafficking and protecting itself. The agency's Miami office, for example, has no identifying sign outside. A van blocks the front door to ward off car-bomb attacks. Inside the van, armed guards wait behind darkened windows.

Parking spaces around the building's perimeter have been blocked to discourage bombing attacks. Newly installed closed-circuit television cameras pan the area. At the street entrance to the parking lot, an armed state wildlife officer -- temporarily reassigned from hunting drug traffickers in the Everglades -- guards the building.

The drug kingpins have escalated their war in recent months:

* In December, about the time plans were completed to extradite four alleged Colombian drug traffickers to the United States for trial, word began circulating that Colombian drug czar Carlos Lehder Rivas had bankrolled one or more hit squads being sent to the United States to kidnap, torture and murder DEA agents. Federal law enforcement sources say that information indicates that some Colombians also are interested in blowing up DEA offices or other federal buildings.

Colombian drug smugglers have announced that they will kill five Americans for every alleged drug trafficker extradited to the United States.

* On Feb. 7, a veteran DEA agent was kidnaped from a busy street in Guadalajara, Mexico. Agent Enrique Camarena Salazar, who had been involved in a successful investigation of local cocaine and marijuana traffickers, is presumed dead. Federal law enforcement officials say they think that the kidnapers are Mexican traffickers with ties to Colombian drug organizations.

* Within the past two weeks, the DEA has received word that Colombian drug czars have placed a price as high as $350,000 on the heads of top DEA officials, including Administrator Francis M. Mullen, who retired Friday, and Deputy Administrator John C. Lawn, whom the Reagan administration plans to nominate as Mullen's successor.

* In November, a car bomb exploded outside the U.S. Embassy in Bogota, Colombia, killing a woman and eventually leading to the departure of 17 U.S. officials and their families from the country. Federal law enforcement officials blame drug traffickers for the bombing.

* Also in November, 19 members of a U.S.-supported program to destroy coca plans in Peru were killed in the Amazon jungle.

Although security has been increased in federal courthouses and U.S. attorneys' offices and in DEA officies here and abroad, most officials say little can be done to protect the agents. Danger is part of their job.

"We are extremely concerned," Lawn said in an interview, "but there's not much you can do to protect agents or their families . . . .We can't run scared because pressure is being brought to bear."

Pete Gruden, the special agent in charge of the DEA's Miami office, said, "We're not giving an inch. It's business as usual. As a matter of fact, it increases our resolve."

Gruden says, however, that he looks over his shoulder more than he used to and that he worries about his family's safety. He also worries a little when he sees motorcycles in traffic. In Colombia, drug murders are often carried out by men on motorcycles who overtake cars in traffic and then open up with machine guns.

Mullen said that, despite concern over the attacks, he sees the situation as a sign that the agency is having a serious impact on Latin American drug organizations. "It's an escalation of the drug-trafficking battle," Mullen said. "They make a mistake when they do it. They'll end up the losers . . . . It's an indication that we've finally gotten to them."

There have been major victories in the past year.

In Mexico, law enforcement officials say Camarena was involved in an investigation that inflicted huge losses on drug traffickers. Because of DEA efforts, one group recently lost $26 million and 6,000 pounds of cocaine.

In November, officials seized 10,000 tons of marijuana plants in a raid in the northern state of Chihuahua.

Drug organizations had signaled their displeasure in October when a DEA agent's car was sprayed with machine gun fire in Guadalajara. Authorities say more than half-a-dozen Mexican policemen have been killed in similar attacks in recent weeks.

In Colombia, where officials historically feared taking on drug smugglers, strikes against traffickers were triggered by the April assassination of Justice Minister Rodrigo Lara Bonilla, who had urged strong drug enforcement.

After the assassination, Colombian President Belisario Betancur Cuartas declared war on cocaine traffickers and vowed to begin enforcing a two-year-old extradition treaty with the United States.

Colombian police say they seized 40,000 pounds of cocaine and its precursor products last year and destroyed 27.5 million coca plants and 262 cocaine labs. The first four Colombians to be extradited to the United States appeared in federal courts here and in Miami last month.

Yet drugs continue to pour into this country. In Miami last month, federal agents seized a $119 million Boeing 747 belonging to Avianca, the Colombian national airline, after discovering nearly 2,500 pounds of cocaine concealed among a shipment of Valentine's Day flowers from Colombia.

Frank Chellino, a DEA agent in Miami, reports that between Jan. 15 and Feb. 15 about 8,000 pounds of cocaine was seized in Miami. That compares with 21,500 pounds in all of 1984. "It looks like we're going to be setting some records this year," Chellino said.

Official estimates indicate that about 60 to 80 tons of cocaine entered the United States in 1983, and the State Department reported that world coca production was up one-third in the last year.

In Mexico, the search for Camarena continues. Several hours after his disappearance, a Mexican pilot who flew occasional missions for the DEA also was abducted at gunpoint. He has not been found.

Mullen and Lawn have complained that Mexico has not cooperated fully in the investigation. Law enforcement sources say witnesses in the case have been threatened -- some by the drug traffickers and others by the Mexican Federal Judicial Police, the Mexican equivalent of the Federal Bureau of Investigation.

Other U.S. law enforcement agencies have rushed to aid the DEA.

After Mullen first encountered reluctance among Mexican officials to conduct raids on hideouts of suspects in the kidnaping, U.S. Customs Commissioner William von Raab ordered extensive searches of persons and vehicles crossing into the United States from Mexico, creating traffic backups of several hours and seriously cutting into tourist traffic into Mexico.

As Mexican authorities and State Department officials began to complain that the operation was harming long-term U.S.-Mexican relations, federal sources say von Raab received a perplexed telephone call from his boss, James A. Baker III, the new Treasury secretary, saying, "Please let me know the next time you plan to declare war on a foreign country."

The border searches were eased but not before the United States received a commitment from the Mexican government to work harder on the Camarena case.