The ruling Sandinista Front opened a "national dialogue" with opposition political parties, unions and business groups today in an effort to resolve their differences over political and economic policy.

The meeting, which represented the first formal talks between the Sandinistas and the opposition since June 1981, lasted five hours and was scheduled to resume Thursday morning. Participants agreed on most arrangements for continuing the talks, although a dispute was left unresolved over whether to permit one of the conservative political parties to participate.

Leaders of 33 political parties, unions, business groups, religious organizations and the Roman Catholic Church met at the Foreign Ministry. They represented the nation's far left as well as five conservative parties that are boycotting national elections Sunday.

The dialogue was viewed as a way to draw the conservatives back into the nation's political life. The opposition newspaper La Prensa today called the dialogue the "last recourse" for such a reconciliation.

But the conservatives have suggested that they might withdraw from the dialogue unless the elections are postponed and restrictions on political liberties are eased. Carlos Nunez, one of the nine top leaders of the Sandinista National Liberaton Front, said after the meeting that the vote "absolutely" would take place as scheduled.

In a separate development, the Defense Ministry charged that a sophisticated U.S. spy plane had flown over Nicaragua this morning and caused sonic booms that "caused justified alarm among our population."

A loud explosion was heard in Managua at about 11:30 a.m., leading to reports earlier in the day from Nicaraguan officials that antigovernment rebels may have set off a bomb. A military spokesman said this evening that the explosion was only the noise of the sonic boom.