SAO PAULO, June 6 (Reuters) - Brazil bought 263,370 metric tons of imported rice for 1.32 billion reais ($250.51 million) in a rare auction on Thursday to prevent a price hike after historic floods in the top producing state of Rio Grande do Sul, national crop agency Conab said.

The volume was lower than the 300,000 tons initially expected. About 36,600 tons will be acquired at a future auction, a Conab official said after the bidding ended.

The government's plan to buy imported rice has faced opposition from growers in Rio Grande do Sul, which supplies about 70% of Brazil's output, and national farm lobbies.

Conab said the rice was acquired at almost 5 reais per kilo. The government will subsidize retail sales at 4 reais per kilo, as announced.

The government could buy as much as 1 million tons of rice to stem what it calls price speculation after the deadly foods in southern Brazil, the crop agency has said.

In June, the price of rice processed by industries in Rio Grande do Sul dropped 1% after rising more than 10% in May, Cepea/Irga data showed.

Rio Grande do Sul's rice farmers' federation Federarroz declined to comment.

Roberto Ghigino, the group's vice-president, told Reuters on Monday importing rice was unnecessary as the state's harvest was almost complete when the deadly floods occurred.

He conceded, however, that around 0.5% of the silos in the central region of the state would have been partly submerged, citing an estimate from the state rice agency Irga, which did not respond to requests for clarification this week.

Although there are no precise estimates of how much stored rice might have been spoiled, Ghigino said the losses would not be significant based on the available data.

The state's central region accounts for 10% of rice production, he said.

($1 = 5.2692 reais) (Reporting by Roberto Samora, Andre Romani and Ana Mano)