Colombia's democratic government, increasingly menaced in recent years by guerrilla warfare, has embarked on an ambitious program to pacify the country by offering insurgents freedom to return to legal activity and a forum for negotiations on political and social reform.

The new peace project, described here as one of the most significant efforts in Latin America to end political violence through conciliation, will begin with a landmark amnesty law now nearing final approval in the Colombian Congress.

The measure would grant unconditional amnesty to members of the country's half dozen extremist groups for nearly all crimes committed during decades of terrorism and warfare with the armed forces. Officials say the new law could result in the release of as many as 400 convicted guerrillas from jail and cause hundreds of others to lay down their arms.

Approval of the measure, which has been endorsed by President Belisario Betancur and overwhelmingly passed by the Colombian Senate, is expected to be followed by negotiations between leaders of major insurgent groups and a special peace commission over legal, social, and electoral changes.

Some major guerrilla leaders, including heads of Colombia's best-known insurgent movement, the M19, have endorsed the proposed law and said they hope to transform their groups into legal electoral parties.

"We look on this as the first political step in a long and difficult journey toward the pacification of the country," said Ramiro Lucio Escobar, an M19 leader who has recently emerged as a spokesman for the group. "We want as much as the government to prevent a Central American-like collapse."

Other guerrilla leaders have rejected the peace process, and the wave of recent violence has convinced some political leaders that Colombia's plans will amount to little more than a good-faith democratic gesture before war resumes between the estimated 4,000 to 6,000 armed insurgents and the Colombian Army.

But government officials, while conceding their project could fail, argued that amnesty and negotiations offer the only real hope of ending decades of political violence in this relatively poor South American nation while preserving a stable democracy.

"This is a country worn down by violence, tired of armed struggle," said Rodrigo Escobar Navia, the minister of government in Betancur's three-month-old administration. "And the fact is that a democratic country has to think more in political solutions to these problems rather than solutions of repression, if it is really to be a democracy."

In pursuing the peace project, Colombian officials say they hope to follow the example of neighboring Venezuela, a democratic country that offered amnesty to its guerrilla insurgents in the late 1960s and that now has former guerrilla leaders serving in its Congress.

But the challenge faced by Colombia, officials say, is closer to that of war-torn Central America. In Venezuela, guerrillas were decisively defeated in the field. They joined a society brimming with oil wealth and economic opportunity.

Colombia's 27 million people remain limited by underdevelopment and traditionally strong social barriers. The underpopulated, often isolated countryside has been torn by political violence since the 1940s.

The Army campaigns have severely weakened the M19, estimated to have 500 to 1,000 members, which took over the Dominican Republic Embassy here in 1980. But the older and larger Revolutionary Armed Forces of Colombia and several other groups remain well entrenched in mountainous or jungled regions.

Estimates of the activity of the groups vary, but the Colombian Permanent Committee for the Defense of Human Rights estimated that Army operations against guerrillas resulted in 2,200 arrests and 267 deaths during 1981, not counting the dozens of soldiers and police killed by insurgents. In addition, committee officials say that nascent right-wing death squads have slain 65 persons during the past three months.

Liberal president Julio Cesar Turbay, who preceded the Conservative Betancur, failed twice to lure the modern leftist guerrilla band out of the field with limited amnesty programs.

The new peace effort began to gain impetus following Betancur's election in June of this year after a campaign in which conciliation with insurgent groups became the leading pledge of all major candidates. The outgoing Turbay government fulfilled a guerrilla demand by lifting the 34-year-old state of siege and the security act that permitted military justice for suspected guerrillas.

The amnesty bill, originally proposed by a prominent socialist senator, dropped almost all of the conditions attached to Turbay's earlier offers, which were spurned by all but a few insurgents. Legislators and defense lawyers now say guerrillas arrested or charged by the military will qualify for freedom in all but a few cases involving capital crimes.

Among the changes being studied by the government, said Government Minister Escobar Navia, are a revamping of laws now governing the predominantly two-party electoral system, government financing of electoral campaigns, a law guaranteeing rights to opposition parties, a reorganization of the civil service to reduce ruling party patronage, and economic development measures for the areas where the guerrillas are now based.

The proposed laws, representing a radical change in Colombia's traditionally elitist democratic system, have gained broad political support.

M19 leader Lucio Escobar, who recently surfaced after completing a three-year prison sentence, said in an interview that "this is a different situation -- we believe the government is sincere."

Despite the long odds of success, many legislators here said that the peace project was an initiative Colombia had to attempt.

"With this historical position, Congress is proving its real will for a peaceful solution," said Liberal Senator German Bula, a sponsor of the amnesty. "We have at least offered an alternative to the violence."