Caracas - Venezuela's intelligence service hauled two prominent opposition leaders back to prison on Tuesday, triggering international outcry as embattled President Nicolas Maduro moved to shore up his power after an election widely denounced as a sham.The raids, carried out in the dead of night, came just one day before a new assembly elected on Sunday is supposed to take office, superseding the opposition-controlled legislature.In a statement, the Supreme Court said protest leader Leopoldo Lopez and Caracas mayor Antonio Ledezma were sent back to prison because they had violated the terms of their house arrest by making political statements.Authorities acted with urgency, it said, because they had received intelligence that the pair "had a plan to flee" - something the men's lawyers vehemently denied.In a video he pre-recorded in case he was sent back to jail, Lopez urged his supporters to keep fighting Maduro's government."If you're seeing this video, it's because they illegally and unjustly came and returned me to prison. I'm a political prisoner," he said."We must not give up the fight. We must never surrender. We must not tire of demanding a better Venezuela."Lopez, the Harvard-educated founder of the Popular Will party, also announced that his wife, Lilian Tintori, was pregnant, calling it "the best news" since he was arrested in 2014, and "one more reason to fight for Venezuela".The men are two of Venezuela's most high-profile opposition leaders. Both had called for a boycott of Sunday's vote for an all-powerful "constituent assembly" tasked with rewriting the constitution.The United States, which has already slapped sanctions on Maduro and top officials, was scathing in its reaction to the latest news.President Donald Trump sternly warned Maduro's "dictatorship" that he holds him personally responsible for the health and safety of the two men."Mr Lopez and Mr Ledezma are political prisoners being held illegally by the regime," Trump said.UN Secretary General Antonio Guterres urged Maduro's administration to "lower tensions" and "find avenues for political dialogue", an appeal echoed by European Union diplomatic chief Federica Mogherini's spokesperson.Spain said it would push for EU sanctions.Lopez and Ledezma were picked up by the intelligence service known by its acronym Sebin, their families said, adding that they held Maduro responsible for the men's lives.Families in the dark "They just took Leopoldo away. We do not know where he is or where they are taking him," Tintori said on Twitter.She released home security camera footage in which four uniformed police officers and three others in civilian garb are seen putting her husband into a car and taking off, with other cars escorting them.Ledezma's family also released a cell phone video in which the mayor is seen being hauled from home in a pair of blue pyjamas as his neighbours scream.Lopez, 46, was transferred to house arrest in July after serving three years and five months in prison as part of a 14-year term. He had been convicted of instigating violence during protests against Maduro in 2014 that left 43 people dead.Ledezma, 62, was arrested in February 2015 on charges of conspiracy and racketeering and was placed under house arrest three months later for health reasons.Opposition lawmaker Freddy Guevara said the re-arrests were aimed at "frightening us and demoralising us".Four months of street demonstrations since April against Maduro have left more than 120 people dead, including 10 over the weekend.The new constituent assembly is to start working on Wednesday. It is made up only of members of Maduro's Socialist party, including his own wife. The opposition has called for protests against the inauguration.'Imperial orders' Maduro has dismissed the US sanctions and criticism, retorting that he will not heed "imperial orders".Latin American nations including Chile, Colombia, Mexico and Peru joined the US in saying they did not recognise the results of Sunday's election, while Brazil, Costa Rica and Panama joined the condemnation of Lopez and Ledezma's arrest.Officials say more than 40% of Venezuela's 20 million voters cast ballots on Sunday.The opposition says turnout was closer to 12% on a par with the population of state employees, who were under major pressure to vote.According to polling firm Datanalisis, more than 70% of Venezuelans oppose the new assembly.