The Reagan administration, with Secretary of State Alexander M. Haig Jr. in the forefront, yesterday hailed the high voter turnout in El Salvador's elections as a defeat for the guerrillas and hinted at new moves to entice the insurgents "to take part peacefully in the democratic process."

Although the outcome of the balloting for a constituent assembly was still unclear, Haig took the unusual step of appearing at the State Department's daily press briefing to read a statement calling the turnout "an unmistakable repudiation of the advocates of force and violence."

He said that for the guerrillas, who had sought to make the elections virtually impossible to carry out, the results were "a military defeat quite as much as a political repudiation. Despite their clear intention to disrupt the elections, the guerrillas were unable to shake either the people or the security forces at their moment of greatest vulnerability."

While other U.S. officials said it was too early to predict what kind of interim government is likely to emerge from the elections, Haig hinted that the administration believes U.S. persuasion and pressure will lead to formation of a coalition pledged to continue U.S.-supported policies of internal reform and resistance to leftist attempts to win power by force.

He said: "We are confident that the constituent assembly, given the extraordinary mandate it has received from the Salvadoran people, will find ways to hold out a hand of conciliation to those adversaries who are prepared to take part peacefully in the democratic process now so encouragingly under way in El Salvador."

Haig did not elaborate. However, it is known that the administration, which gambled heavily on the belief that the elections would demonstrate a lack of popular backing for the guerrillas, now hopes that a new effort can be made to convince the insurgents that their only hope for a share of power lies in winning support through the ballot box rather than on the battlefield.

Although the details are secret, the administration has been preparing "suggestions," involving guarantees of protection and other enticements that it believes might provide the basis for negotiations between the government that emerges from the elections and the Salvadoran left.

In addition, the administration is expected to put increased pressure on the guerrillas to negotiate by launching a new effort to cut them off from the support they allegedly receive from Cuba and Nicaragua. Haig, calling attention to that requirement, said: "The guerrillas still have the external support to continue their campaign of terror at levels that would be impossible if they depended on their own people."

Last Friday, the State Department denied a Mexican announcment that plans have been made for U.S. and Nicaraguan officials to meet in Mexico City in early April. Department spokesman Dean Fischer yesterday reiterated that talk of such a meeting was "premature."

In private, U.S. officials said the administration is unwilling to commit itself publicly to discussions with Nicaragua before the new Salvadoran government is in place and Washington has a clearer picture of how serious the Nicaraguans are about meaningful negotiations. But, the officials added, while the administration remains doubtful about the chances for successful talks, it is committed to give them a try soon because of the need to show congressional and foreign critics of President Reagan's policies that it is willing to explore any avenues that might resolve tensions in Central America.

On Capitol Hill, the first reactions to the voter turnout were positive, although most members of Congress reserved judgment on what the outcome might mean for future U.S. involvement in the Salvadoran conflict.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who had advocated a go-slow approach in aiding El Salvador until after the elections, said the vote showed that "the masses down there appreciate the freedom of elections." He added: "I just hope they can bring a majority party to power that can bring all factions together."

Although some Reagan supporters hailed the result as a potential turning point, most agreed with the more cautious assessment of House Minority Leader Robert H. Michel (R-Ill.), who said: "No one can say what will happen next, but history will record that for one brief shining moment, amidst the darkness of war, the people at least voted in large numbers."