BOGOTA, COLOMBIA, JAN. 27 -- The Colombian government, trying to curb a nationwide escalation of political and drug-related violence, today decreed new antiterrorist statutes promising tougher law enforcement and more severe penalties against offenders.

The measures, which broaden the arrest powers of security forces and establish special police and judicial units to handle terrorist cases, are the boldest steps taken so far to combat the combined threats of cocaine traffickers, leftist guerrillas and right-wing death squads.

In the face of growing public concern that the violence may have exceeded the government's ability to quell it, the moves represent an attempt by the administration of President Virgilio Barco to reassert its authority and restore public security.

The assassination Monday of the attorney general by gunmen said to be working for cocaine traffickers triggered the crackdown, but the government has been under pressure for months to take stronger steps against lawlessness. January has been the most violent month in Colombia since a civil war between the Liberal and Conservative parties ended in 1957.

Attacks on military and commercial installations by leftist guerrillas and killings of leftist politicians by right-wing groups have intensified, along with what appears to be a new offensive by the country's powerful cocaine traffickers to intimidate or weaken the government.

While there still seems little danger of the government being toppled in a military coup after three decades of civilian rule, many have begun to sense a worsening of the security situation.

"Clearly, something quite different, quite worrisome, has been happening," said a foreign diplomat.

The measures announced today, however, were not as sweeping as some people had hoped and reinforced doubts that the government would be able to overpower criminal forces. Some jurists said the decrees failed to address the root causes of Colombia's violence or do anything about corruption in political, judicial and law enforcement circles.

Of the urpsurge in assaults, most alarming have been those by the drug traffickers in the past week on two of the nation's most respected political figures. Although the exact motives of the killing of Attorney General Carlos Mauro Hoyos Jimenez and the kidnaping of Bogota mayoral candidate Andres Pastrana are unknown, the actions are widely believed to reflect the drug barons' determination to head off any new government attempt to extradite them to the United States.

Murders of dozens of judges and death threats against many others over the past few years are considered to have undermined the ability of local courts to try the traffickers. The Reagan administration has repeatedly urged that leaders of the so-called Medellin Cartel, reputed to supply from 70 to 80 percent of cocaine shipped to the United States, be handed over for trial on an array of criminal charges.

But the new antiterrorist measures say nothing about restoring extradition. Colombia's Supreme Court, acting under the shadow of death threats, ruled last June that a 1979 U.S.-Colombian treaty that had been used to extradite more than a dozen traffickers could no longer be applied because its implementing legislation had been adopted improperly. As a result, extraditions to the United States have been suspended.

U.S. officials are still hoping the Barco government can find a way to reinstitute extradition, possibly under older international conventions. But public opposition to sending Colombian nationals abroad for trial is mounting among many here who view extradition as an affront to national sovereignty and an invitation to more attacks by drug traffickers.

The possibility, meantime, that U.S. forces might be asked to intervene in Colombia -- as they were against traffickers in Bolivia in July 1986 -- has been ruled out, according to U.S. officials who were told by Colombian authorities that such intervention would be politically intolerable.

In preparing the new measures, Colombian officials said they studied antiterrorist codes in such West European countries as Spain, Italy, West Germany and Britain. Promulgated under the government's existing state-of-siege powers, the decrees raised the maximum jail term to 25 years for convicted terrorists. They empowered security forces to arrest suspected terrorists without judicial orders and made those who withhold information about a terrorist act subject to prosecution.

Among the law-enforcement moves will be closer control of helicopters, which traffickers have frequently used. Pastrana disclosed after his rescue Monday that a helicopter had been used to fly him out of Bogota, where he was kidnaped Jan. 18, to a hideaway near Medellin.

Under the new orders, news media will be prohibited from reporting the testimony of witnesses to certain terrorist actions in order to protect the witnesses.

To bolster an overburdened judicial system, the government is creating 4,990 new judiciary posts. It is also setting up a fund to offer financial rewards to informers.

To close a legal avenue used by traffickers to spring themselves from jail, habeas corpus pleas in the future will be heard not by district judges but by higher level magistrates. This way, officials hope to exercise closer control over who is released. In December, Jorge Luis Ochoa Vasquez, reputed to be world's second richest cocaine trafficker, was freed from a Bogota prison by a low-ranking judge on a habeas corpus plea.

With municipal elections due in March, some of the latest rise in violence had been anticipated. The vote will mark the first time that mayors, previously appointed, will be elected. It is being heralded as a major step toward opening up a political system that for three decades has rested in the hands of the Liberal and Conservative parties.

Leftist candidates belonging to the fledgling Patriotic Union are being gunned down in towns around the country by groups opposed to the reform.