One of El Salvador's largest leftist groups declared a halt to violent acts against the new military government here today in response to promises by the junta to legalize the Communist Party and bring leftists into the government.

The Popular League-28, a coalition of student, union and peasant groups, said it was encouraged by junta promises in a news conference yesterday that its organization and other mass groups will be called on to partcipate in the new government and that the Communist Party will be legalized. It described such promises as "superficial, but a good sign."

Meanwhile, the five-man junta held an emotional meeting with human rights groups and families of missing persons who told them confidence in their government depends on immediate action to locate secretly held political prisoners and punish their torturers.

The civilian-military government that replaced President Carlos Humberto Remero after a coup Monday is trying to gain the confidence of El Salvador's violence-weary people. To do that, the government has acknowledged, it will have to fulfill quickly its initial promises for social and economic change and deal with extremist attacks from the right and left that helped cause Romero's downfall.

The United States, which had been monitoring the increasingly violent situation in El Salvador before the coup, had been worried about the possibility of a leftist takeover. The Carter administration said Tuesday that it was "encouraged" by the new government.

So far, the junta has not been spared violence from the left, or even from the government's own security forces, most of whose membership remains the same as under Romero.

Three bombs, set by the leftist People's Revolutionary Army, went off in the capital last night, destroying two electrical substations and cutting power to parts of the city.

This morning, a retired Army colonel was shot as he left his home and seven public buses were burned by the People's Revolutionary Army. At midday, two persons, reportedly students, were shot dead outside the National University. Initial reports blamed the government security forces.

Violence in the past 24 hours followed several confrontations between leftists and the military during the first two days under the new government. At least 16 were reported killed when the military used armored vehicles and heavy weapons to dislodge protesters who had occupied suburban municipal buildings and striking workers in four factories.

In a press conference this morning, the Popular League-28, one of three coalitions aligned with separate guerrilla groups, said that "what the new government has said until now reflects good will. We are not going to be violent against the junta, but we will continue to organize the masses, watching to make sure the government carries out its promises."

The group's political commission said it was disturbed over what it called a "military reaction to political events" in the confrontations earlier in the week.

It said that last night's bombs and today's bus burnings were carried out by the People's Revolutionary Army, the guerrilla "armed wing" of the league. While such actions would be avoided in the future, the representatives said, those had already been planned for some days and could not have been stopped.

Today's Popular League statement was an about-face from one issued yesterday in which the organization denonced the junta and called its three civilian members "traitors."

Of the two other mass action groups, the Popular Revolutionary Bloc had condemned the junta as a "military maneuver" and the other group had made no public comment.

The junta press conference yesterday also raised the hopes of the families of hundreds of Salvadorans who have disappeared at the hands of security forces in the past several years.

Yesterday, the junta invited members of the Salvadoran Human Rights Commission and a group called "Mothers of the Disappeared" to enter and investigate all jails in the country to look for those missing and believed to be political prisoners.

Members of those groups met today with the junta and refused to make their own investigation of the jails. Instead, they demanded government action.

"You have the resources, you have the power," one mother said.

"We cannot go to the jails," said another. "We cannot take it. We haven't the will to visit the places where our children have been tortured."

Although the Romero government denied secretly holding political prisoners, local human rights and church agencies have documented nearly 300 cases of persons taken away by security officials in the past six years. According to San Salvador's archbishop, 196 disappeared during the 28 months of the Romero administration.

In a report scheduled for release Monday, the Organization of American States' Human Rights Commission charged the rightist Romero government with both torture and secret imprisonment in small "dungeons."