President Donald Trump came close to acknowledging his election loss on Sunday morning, claiming his rival Joe Biden "won" through fraud, before quickly declaring that he would continue to challenge the result. "He won because the election was rigged. No vote watchers or observers allowed, vote tabulated by a radical left privately owned company," Mr Trump tweeted in his clearest admission of defeat so far. But an hour and a half later he fired off two tweets in quick succession saying, "Rigged election. We will win!" and "I concede nothing! We have a long way to go." Mr Trump has refused to concede the election, and repeatedly said he intended to overturn the result through legal cases. No evidence of mass fraud in the November 3 election has been found, and legal challenges crumbled in several states on Friday. In total, the president’s campaign has lost more than a dozen legal challenges. Mr Trump has encouraged his supporters to protest against the alleged electoral fraud, and thousands rallied in Washington on Saturday as part of a nationwide "Million MAGA March" to back his claims. Clashes with rival protesters broke out in the evening, with at least one stabbing reported. Twenty protesters were arrested for public disorder. Several other cities on Saturday faced gatherings of Trump supporters unwilling to accept Mr Biden’s Electoral College and popular vote victory as legitimate. Cries of “stop the steal” and “count every vote” continued in spite of a lack of evidence of voter fraud or other problems that could reverse the result. After night fell, the relatively peaceful demonstrations in Washington turned from tense to violent. Videos posted on social media showed fist fights, projectiles thrown and clubs used as supporters of Mr Trump clashed with those demanding they take their banners and leave. The tensions extended to Sunday morning. A variety of charges, including assault and weapons possession, were filed against those arrested, officials said. Two police officers were injured and several firearms were recovered by police. Mr Trump had given an approving nod to the gathering on Saturday morning by driving his motorcade through streets lined with supporters before rolling on to his Virginia golf club. People chanted “USA, USA” and “four more years,” and many carried American flags and signs to show their displeasure with the vote tally. “I just want to keep up his spirits and let him know we support him,” said one loyalist, Anthony Whittaker of Winchester, Virginia. He was outside the Supreme Court, where a few thousand assembled after a march along Pennsylvania Avenue from Freedom Plaza, near the White House. A broad coalition of top government officials declared that the November 3 voting and ensuing count unfolded smoothly with no more than the usual minor hiccups. “The most secure in American history,” they said, repudiating Mr Trump’s efforts to undermine the integrity of the contest. In Delray Beach, Florida, several hundred people marched, some carrying signs reading “Count every vote” and “We cannot live under a Marxist government”. In Lansing, Michigan, protesters gathered at the Capitol to hear speakers cast doubt on results that showed Mr Biden winning the state by more than 140,000 votes. Phoenix police estimated 1,500 people gathered outside the Arizona Capitol to protest against Mr Biden’s narrow victory in the state. Protesters in Salem, Oregon, gathered at the Capitol building. Among the speakers in Washington was a Georgia Republican newly elected to the US House. Marjorie Taylor Greene, who has expressed racist views and support for QAnon conspiracy theories, urged people to march peacefully to the Supreme Court. The marchers included members of the Proud Boys, a neo-fascist group known for street brawling with ideological opponents at political rallies. Confrontations occurred later in the day as small groups of Trump supporters attempted to enter the area around Black Lives Matter Plaza, about a block from the White House, where several hundred anti-Trump demonstrators had gathered. In a pattern that kept repeating itself, those Trump supporters who approached the area were harassed, doused with water and had their Maga (Make America Great Again) hats and pro-Trump flags snatched and burnt, amid cheers. As night fell, police lines kept the two sides apart. Videos on social media showed some demonstrators and counterdemonstrators trading shoves, punches and slaps. A man with a bullhorn yelling “Get out of here!” was shoved and pushed to the street by a man who was then surrounded by several people and shoved and punched until he fell face first into the street. Bloody and dazed, he was picked up and walked to a police officer. The “Million Maga March” was heavily promoted on social media, raising concerns that it could spark conflict with anti-Trump demonstrators, who have gathered near the White House in Black Lives Matter Plaza for weeks. In preparation, police closed off wide sections of downtown, where many shops and offices have been boarded up since election day.