NEW DELHI — India's top court on Thursday stopped the government of Tamil Nadu state from releasing three of the seven prisoners serving life sentences for the 1991 assassination of former Prime Minister Rajiv Gandhi, a lawyer said. The state ruled Wednesday that the seven should be released because they have served more than 20 years in prison. Critics immediately slammed the decision, calling it a transparent attempt to win over Tamil voters in this year's national elections. The federal government petitioned the Supreme Court on Thursday to stop the state from freeing the prisoners, with Prime Minister Manmohan Singh saying the move was “not legally tenable.” The court on Tuesday had commuted the death sentences for three of the convicts, after their lawyers argued that executing them now, after they had already served long prison terms, would amount to an unconstitutional double punishment. Rakesh Dwivedi, a lawyer for Tamil Nadu's government, said the court's order Thursday applied only to the three prisoners whose sentences had been commuted, and that the state was free to release the other four. — AP