Virgilio Barco Vargas, a political moderate of the Liberal Party, won a decisive victory today in Colombia's presidential election, unseating the Conservative Party.

Barco swept to victory on a platform promising greater firmness against leftist guerrillas, less fanfare over the Contadora peace process in Central America and more welfare assistance for the poor.

With more than 70 percent of the ballots counted, the results showed Barco, a U.S.-educated engineer with a long career in government, with an insurmountable 24-point lead over Conservative Party candidate Alvaro Gomez Hurtado, who spent his campaign countering his hard-line image.

Gomez conceded defeat, saying in a television statement: "The figures . . . show the next president will be Virgilio Barco."

Behind the Liberal leader's victory were not only Barco's reputation as an able administrator but also fears of what a Gomez presidency might bring and public discontent with the unfulfilled promises of outgoing President Belisario Betancur, a Conservative.

Soldiers and police were mobilized nationwide to guard against terrorist attacks during today's vote. In recent weeks, Colombia's various guerrilla forces have stepped up bombings of party headquarters, ambushes of military patrols and raids on small towns. The attacks are taken as a sign of the unsettled relations between the traditional political establishment and leftists seeking greater public participation in government and economic aid for lower income groups.

The heavy show of security did not appear to dampen the day's festival atmosphere as ballots were cast in voting places surrounded by party stands colorfully adorned with flags and posters.

Betancur, a politician with a populist touch and genial manner, dealt with the country's chronic warfare and poverty by signing cease-fires with most of the guerrilla groups, pledging urban and agrarian reforms, and cracking down on some drug operations. But an economic recession, more than two years of rigid austerity and difficulty until last year in obtaining foreign credits frustrated his designs. He was prohibited by law from running for another term.

One positive legacy of Betancur's four-year term has been a prolonged truce agreement with the Revolutionary Armed Forces of Columbia (FARC), the oldest and largest guerrilla group, which entered a candidate in today's race -- former judge Jaime Pardo Leal -- under the banner of the Patriotic Union, a political movement set up last year by FARC and the Communist Party.

Barco, who appeared assured of victory early in the campaign, stuck to generalities and avoided details in speeches aimed at uniting and mobilizing the Liberals, Colombia's largest party. Though a poor speaker, the 64-year-old white-haired, bespectacled politician grew more at ease on the stump as party loyalists poured out for huge rallies in anticipation of regaining power from the Conservatives.

He is married to an American, the former Caroline Isakson, of York, Pa.

Methodical and demanding, Barco established a reputation as a good manager in previous posts as mayor of Bogota and as minister of public works and agriculture. He has also served as ambassador to London and Washington.

Barco has indicated that after he takes office Aug. 7, he will be less tolerant of cease-fire abuses than Betancur was and will attempt to coordinate his policy more closely with the military. At the same time, Barco is said to be preparing offers of rural development and social reform to persuade all guerrilla groups to lay down their arms.

In international relations, he is known to favor a lower profile for Colombia in the Contadora group, which Betancur helped establish, and to give more attention to relations with Colombia's immediate neighbors.

Gomez, 67, a lawyer and former journalist who also was Colombia's ambassador to Washington, faced the delicate task of living down his political past. His candidacy aroused memories of the political violence in Colombia in the late 1940s and 1950s when Laureano Gomez, the candidate's father, was president.

Gomez had initially planned this time to run a low-key campaign, counting on a split in the Liberals caused by the short-lived candidacy of Luis Carlos Galan, a progressive-minded politician who broke away from the party and eventually withdrew from the race.

As the clear underdog against a reunited Liberal Party, Gomez decided to go on the offensive. He made promises to groups normally outside Conservative ranks, offering discounted bus fares to students, low-cost housing to the poor and removal of the ban on divorce in Catholic marriages.

Heated exchanges between the major parties marked the final days of the campaign as Gomez and his supporters questioned the legality and morality of Barco becoming president, given his family's involvement in a lawsuit against the government.

With private polls showing him behind Barco by about 20 percent, Gomez dropped a political bombshell last Monday night in a 20-minute TV broadcast highlighting the lawsuit.

The case stems from the royalties the Barcos were granted starting in 1931 as compensation for the takeover of oil-rich territory discovered in 1905 by Virgilio Barco, the candidate's grandfather.

Present-day holders of the Barco shares, including Barco's three daughters, two brothers and about 30 relatives, are now asking the equivalent of $18.5 million in compensation from the Colombia Petroleum Co. and Ecopetrol for a 1974 decision impeding the sale abroad of 86,091 barrels of oil.

Barco sold his share of the royalties in 1968.

But Gomez claimed that since members of Barco's family are suing the nation, Barco's role as president would be compromised.

The prosecutor general ruled last Thursday that the lawsuit would not disqualify Barco from exercising his presidential responsibilities. The Council of State, which is independent of the executive branch, will also rule on the matter.