BRASILIA, Sept 6 (Reuters) - Brazil prepared for Independence Day demonstrations on Tuesday by supporters of far-right President Jair Bolsonaro, who has called for a show of support for his attacks on the country"s Supreme Court that are rattling Latin America"s largest democracy. Security was reinforced in the capital, Brasilia, and police started blocking access to the central mall where thousands are expected to march, supporting Bolsonaro in his clash with the judiciary over changes to Brazil"s voting system. Critics fear the president is encouraging supporters to the point they might try to intimidate or invade the courts, warning of parallels with the storming of the U.S. Capitol by supporters of then-President Donald Trump after he gave a fiery speech falsely claiming his election defeat was the result of fraud. Internationally, more than 150 left-leaning former presidents and party leaders signed an open letter criticizing Bolsonaro for encouraging what they called an imitation of the Jan. 6 assault on the U.S. Capitol. The demonstrations are "stoking fears of a coup d"état in the world"s third-largest democracy," the letter warned. Bolsonaro said on Friday the demonstrations would be an "ultimatum" to the Supreme Court justices who have taken "unconstitutional" decisions against his government. The court has authorized investigations of Bolsonaro allies for allegedly attacking Brazil"s democratic institutions with misinformation online. He has called the court-ordered probes a violation of free-speech rights. Congress and the courts also resisted Bolsonaro"s attempt to introduce paper voting receipts as a backup of an electronic voting system, which he argues is vulnerable to fraud. The electoral court maintains the system is transparent and safe. Bolsonaro"s critics say he is sowing doubts so he can challenge the results of next year"s election, which opinion polls now show him losing to former leftist President Luiz Inacio Lula de Silva. Neither has confirmed his candidacy. In a brief speech to the nation on social media, Lula said the president on Independence Day should be offering hope of economic recovery and solidarity with the victims of COVID-19. "Instead of announcing solutions, he is calling people to confrontation ... in acts against democracy," Lula said, accusing Bolsonaro of sowing "division, hatred and violence." Brasilia will have 5,000 police and military personnel on hand to maintain order at the march outside Congress, where Bolsonaro is scheduled to appear on Tuesday morning. In the afternoon, Bolsonaro will join supporters on a major avenue in midtown Sao Paulo at a gathering he has forecast to be the biggest political rally in Brazilian history. Many leftist leaders have urged their followers to avoid clashes by avoiding counter-demonstrations on Tuesday in favor of larger anti-Bolsonaro protests on Sunday. Financial, industrial and agribusiness leaders in Brazil have also distanced themselves from Bolsonaro"s attacks on the Supreme Court and called for dialogue between the executive and judiciary, warning the tensions threaten an economic rebound.