The United States yesterday dismissed an offer by Nicaragua's leftist Sandinista government of amnesty for U.S.-backed "contra" rebels as "meaningless" without a "political opening" that would allow opposition forces to participate safely in the political life of the country.

The amnesty offer was made Thursday by Nicaraguan President Daniel Ortega as he was inaugurated. However, the United States has denounced the Nicaraguan presidential election Nov. 4 as a sham because parties opposing the Sandinistas were not allowed to campaign freely. State Department spokesman Alan Romberg said that "an amnesty without a genuine political opening would be meaningless." He added that the United States has "seen little or no indication of actions taken by the Sandinistas" to achieve the four goals that the Reagan administration has set as the price for better relations with Nicaragua.

These goals call for reduction of Nicaragua's military forces, removal of Soviet and Cuban military personnel from Nicaragua, an end to Nicaraguan support of leftist subversion in other Central American countries and establishment of pluralistic democratic processes.

Romberg also called attention to a letter sent to Ortega by Carlos Andres Peres, former president of Venezuela and a vice president of the Socialist International. In the letter, partially republished yesterday by The Wall Street Journal, Peres declined Ortega's invitation to attend his inaugural "because sufficient guarantees were not provided to assure the participation of all political forces" in the Nicaraguan election.

Peres, who said he was speaking as a friend of the Sandinista revolution and a foe of outside intervention there, called for continued movement toward democracy and concluded, "I continue to express confidence that . . . under your presidency, you will follow paths that definitively and authentically clear up the confusion and doubts surrounding the goals of the Nicaraguan revolution . . . ."