A calm Sunday break in the Organization of American States' General Assembly here was shattered this afternoon when Grenadan government troops broke up a local political rally.

At least one person was arrested and two were injured in the action, during which some 25 Grenadian soldiers fired rifles into air and hit several people with clubs.

The gathering of about 1,000 persons took place in downtown St. George's, three miles from where the week-long OAS conference, attended by the foreign ministers of more than 20 Latin American nations, is being held. Secretary of State Cyrus Vance left the conference Thursday.

Organized by leaders of the political opposition to Grenadan Prime Minister Eric M. Gairy, the rally was billed asa protest against human rights violations in this tiny Caribbean nation. Human rights in the Americans has been the main issue at the OAS confrence, which began last Tuesday.

Except for one brief period during the 1960s, Gairy, 55, has controlled issued politics since 1951. Following independence from Great Britian in 1974, he was chosen prime minister. The opposition, led by local attorney Maurice Bishop, 33, charged fraud in the last election, and has accused Gairy of political corruption and of allowing foreign investors to control the island economy.

An anti-Gairy strike in early 1974 shut down the island, whose population is less than 120,000 for an entire month and resulted in soldiers killing at least one person. Today's rally was clearly an opposition attempt to gain publicity for its cause and to capitalize on the presence of an international press corps here for the OAS conference. Yesterday, the government issued a statement accusing Bishop of breaking conference "security lines" by visiting reporters who invited him to their hotels and of spreading "malicious propaganda."

The soldiers appeared within minutes after today's rally began, cut wires leading to loud speakers and ordered the crowd to disperse. The shots began several minutes later.

Soldiers confiscated film from two American wire service photographers at gunpoint. About a dozen foreign reporters witnessed the incident, and two were accused of holding a destine meeting with Bishop.

Gairy opened the OAS conference Tuesday by telling delegates that his country was one in which human and legal rights had always been respected.