BRASILIA/SAO PAULO, Sept 7 (Reuters) - Brazilian President Jair Bolsonaro slammed the country"s Supreme Court and cast doubt on the integrity of next year"s elections on Tuesday as his supporters rallied in major cities at a time of heightened tensions in Latin America"s largest democracy. Facing slipping poll numbers, surging inflation, and criticism for his handling of the world"s second-deadliest coronavirus outbreak, Bolsonaro has urged supporters for weeks to protest his perceived enemies in Congress and the courts. More than 100,000 supporters turned out in Sao Paulo, according to state security officials – far short of the record turnout Bolsonaro forecast, but perhaps enough to embolden the president in his standoff with the judiciary and Congress. "We cannot accept a voting system that does not offer any security in the elections," Bolsonaro said in Sao Paulo, repeating a demand for paper voting receipts blocked by Congress and the federal electoral court. "I can"t participate in a farce like the one sponsored by the head of the electoral court." Bolsonaro"s critics say he is sowing doubts so he can challenge the results of the 2022 presidential race, which opinion polls now show him losing dramatically to former President Luiz Inacio Lula da Silva. Neither has confirmed his candidacy. Bolsonaro also ramped up criticism of the Supreme Court for authorizing investigations of him and his allies, based on accusations that they had attacked Brazil"s democratic institutions by promoting misleading information on social media. The president has derided the probes as violations of political freedoms. The Brazilian president has often drawn comparisons with former U.S. President Donald Trump, who he has said he admires. Jason Miller, a former Trump adviser and conservative social network entrepreneur got caught up in the drama in Brasilia on Tuesday when he was detained and questioned for three hours by Brazilian police as part of the probes. A lawyer for Miller, who had attended the Conservative Political Action Conference summit organized by one of Bolsonaro"s sons, said he chose to remain silent. The scenes at major rallies in Sao Paulo, Rio de Janeiro and Brasilia were mostly festive, with government supporters dressed in yellow and green waving flags and cheering. But an undercurrent of frustration was clear in banners calling for military intervention and the dismissal of the Supreme Court. "The military needs to remove those that aren"t letting our president govern ‒ in the Supreme Court, in the Senate, all of them," said 70-year-old retiree Maria Aparecida, on Sao Paulo"s Avenida Paulista. "The Supreme Court doesn"t protect the constitution, so our military must." The president"s criticism of Brazil"s electronic voting system had clearly connected with die-hard supporters, many of whom were convinced of Bolsonaro"s inevitable re-election. "If he loses, we know there was fraud," said Monica Martins, a 51-year-old lawyer at the rally in Rio. Bolsonaro embraced the occasion, donning the presidential sash at a military event marking Independence Day in Brasilia before touring the early rally there by helicopter. He flew midday to Sao Paulo for his defiant address to supporters. "I"ll say to those who want to make me unelectable in Brasilia: Only God will get me out!" he shouted. "And tell the scoundrels that I"ll never be jailed!" Sao Paulo"s Public Security Secretariat estimated that the pro-Bolsonaro demonstration on Paulista Avenue had drawn some 125,000 people, most of whom dispersed quickly after Bolsonaro"s remarks. Many leftist leaders have urged their followers to avoid clashes by skipping counter-demonstrations on Tuesday in favor of larger anti-Bolsonaro protests on Sept. 12.