Brazil has demanded U.S. clarification of President Reagan's statement during his televised address Sunday night that Nicaragua's Sandinista government is aiding radicals in this country.

Acting Foreign Minister Paulo Tarso Flecha de Lima yesterday summoned the U.S. Embassy charge d'affaires to request clarification of Reagan's remarks on the grounds that Brazil has had no internal security problems since at least 1979, when Nicaragua's Sandinista government came to power. Guerrilla groups were effectively wiped out a decade ago.

Reagan said that Brazil, Colombia, Ecuador, Chile, Argentina, Uruguay and the Dominican Republic were victims of Nicaragua's expansionist plans, because radical groups from these countries received training, safe haven, false documents and, occasionally, weapons from the Sandinistas.

A spokesman at the U.S. Embassy in Brasilia said that the Foreign Ministry requested clarification of what the president had meant. The spokesman said the text did not imply that all the countries had received the support from Nicaragua described by the president.

The incident follows a diplomatic exchange between Brazil and the State Department after Secretary of State George P. Shultz also mentioned Brazil as a victim of Nicaraguan terrorism in testimony to a foreign affairs committee on Capitol Hill on Feb. 28.

Elliot Abrams, assistant secretary of state for Latin American affairs, wrote his Brazilian counterpart an "explanatory letter" in which he acknowledged that the U.S. administration was aware that no guerrilla movements existed in Brazil.

Police in Rio de Janeiro, meanwhile, seized a secret cargo of arms aboard a Panamanian-registered freighter during the weekend. The freighter, Nobistor, arrived in Rio from Buenos Aires and carried machine guns, revolvers, rifles and grenades.

The crew of 10 Argentinians and eight U.S. citizens who told local police they were Vietnam War veterans working as mercenaries, said the cargo was picked up in Argentina. First reports said the cargo was destined for the African country of Ghana. Police denied other reports that the arms were destined for Central America, possibly Nicaragua.