Venezuela headed toward a change in government yesterday as Social Christian party candidate Luis Herrera Campins, 53, appeared virtually assured of victory in the presidential race against Luis Pinerua of the ruling Democratic Action Party.

Unofficial media and party tallies, compiled through open counts at individual polling booths throughout the country, gave Herrera a lead of 3-to-5 percent over Pinerua.

Only 10 percent of the votes had been counted officially by last evening and final results may not come until the end of the week. Although Pinerua is not expected to concede the race until today, workers of his Democratic Action Party acknowledged, unofficially, that he had lost. Polls last week had shown the two neck and neck.

Turnout in the Sunday election, Venezuela's fifth since the overthrow of the country's last military dictator 20 years ago, was estimated at about 90 percent of 6 million eligible voters. Many polling places remained open several hours past the official 4 p.m. closing time to accommodate long lines. No violence was reported.

Herrera, whose campaign slogan was "enough already," promised to reorder what he had described during the race as the free-spending policies of President Carlos Andres Perez, to lower the urban crime rate and to improve public services.

He is not expected to make radical changes in the foreign policy of the oil-rich nation, or to alter its position as a moderating force but one seeking oil price increases within the Organization of Petroleum Exporting Countries.

Herrera, whose party is known by its acronym COPEI, will begin his five-year term in the Miraflores Presidential Place in March. Perez was constitutionally forbidden from running again for the next 10 years, although Venezuelan law designates all ex-presidents senators for life.

Through most of Venezuela's last two decades of democracy, COPEI has served as the loyal opposition, with only one winner out of four previous races: Rafael Caldera, who served from 1969 to 1974.

COPEI and Democratic Action are by far Venezuela's largest parties and control national politics in much the same manner as do the U.S. Republican and Democratic parties. There are few noticeable differences between them, with COPEI perhaps concentrating more on domestic social issues.

Both leading candidates spent heavily during the campaign, which officially began in April but unofficially has gone on for nearly two years. Both also followed a growing Venezuelan tradition, begun by Perez in his 1973 race. of hiring U.S. specialists to manage their campaigns.

Herrera recruited David Garth, who worked on the successful reelection race of New York Gov. Hugh Carey, while Pinerua employed Joseph Napolitan, who ran Perez successful 1973 bid for the presidency.

Campaign expenditures for all candidates this year, including eight from small parties, totalled an estimated $100 million, with the two front runners spending the vast majority.