Tens of thousands of anti-government protesters have returned to Venezuela’s streets to decry Nicolás Maduro’s alleged attempt to steal Sunday’s election, as the country’s authoritarian leader vowed to squash what he called “a violent counter-revolution” and more than 700 arrests were made. Maduro’s disputed claim to have won the vote has plunged the South American country into another chapter of unrest and uncertainty which has spooked regional governments. “I cannot say that I am relaxed. I’m not. I am worried. I am leaving here worried,” Celso Amorim, the envoy of Brazil’s president, Luiz Inácio Lula da Silva, told the Guardian on Tuesday morning as he prepared to fly out of Caracas after meeting Maduro the previous day. On Monday, thousands of residents of poor communities once loyal to the Chavista revolution marched through Venezuela’s capital, Caracas, in a striking demonstration of the widespread anger sparked by Maduro’s claim to have beaten his rival, the ex-diplomat Edmundo González Urrutia. Maduro has said he won the election with more than 5.1m votes to his rival’s 4.4m. But the opposition insist they won a landslide, with 6.2m votes to Maduro’s 2.7m. On Tuesday, the demonstrators were back after González and his key backer, the prominent opposition leader María Corina Machado, called on followers to continue their protests. “It’s obvious that we won … We crushed them. 70% of the country is against the government,” claimed one of those to answer their call, a 35-year-old administrator called Ana Maria González. González celebrated how so many residents of low income communities were turning out to challenge the alleged power grab. “The barrios came down,” she said, using the local word for Venezuela’s poor hillside favelas. “This hadn’t happened for so long.” Another protester, the human rights activist Leída Brito, 65, shed tears as she spoke of her fury at the alleged fraud. “We want political change and we voted for it … but Nicolás Maduro and the electoral council have robbed us – they have stolen our dreams. They have stolen the dream of our children being able to come back to Venezuela,” Brito said of the historic exodus that has seen about 8 million citizens flee abroad, including her children who live in Chile and the Dominican Republic. Thousands of anti-Maduro motorbike riders – mostly from the barrios – swept through the streets of Caracas honking their horns and waving flags. “This is Petare representing! We are coming down [from the hills] and the government is going down!” shouted one rider as the tsunami of vehicles pushed ahead. As public anger grew and Caracas residents began stocking up on food for fear of even greater turmoil ahead, there was also mounting international condemnation. The Organization of American States released a scathing report from its election observation department, which accused Maduro’s regime and Venezuela’s government-controlled electoral authority, the CNE, of employing a “coordinated strategy” to ignore, deceive and defraud voters. “The evidence reveals an attempt by the regime to ignore the will of the majority expressed at the ballot boxes by millions of Venezuelan men and women,” the report said, describing an array of “illegalities, vices and bad practices” during the electoral process, which culminated in the CNE handing Maduro a deeply suspect victory. “Nicolás Maduro’s regime has once again betrayed the Venezuelan people by promising to respect popular will when, at the same time, it was doing everything possible to manipulate and ignore that will,” the report said, arguing that the result “deserve neither to be trusted, nor to be democratically recognised”. Speaking to the Guardian on Monday night, Machado urged Maduro to accept that his 11-year rule – during which Venezuela has suffered a catastrophic economic and humanitarian meltdown he blames on US sanctions – was over. “He should understand that he was defeated,” Machado said, rejecting Maduro’s claim that his victory was “irreversible”. “I would say his departure is irreversible,” she said. Under growing pressure at home and abroad, Maduro’s administration has pushed back, describing the protests as a foreign-backed far-right conspiracy to overthrow his government and force the Chavista movement from power after 25 years. Jorge Rodríguez, a close Maduro ally who is president of the national assembly, summoned supporters from the impoverished barrios to march on the Miraflores presidential palace on Tuesday afternoon “to celebrate our victory” and rally behind the embattled president. Security forces, meanwhile, went on the offensive, with masked men filmed detaining Freddy Superlano, a key opposition figure who backed González’s presidential bid. The attorney general, Tarek William Saab, announced the arrest of 749 “delinquents” who he blamed for injuring 48 police officers and soldiers as they sought to promote a “bloodbath”. He said one soldier had been killed. In a video message, González denounced the crackdown: “Regrettably in the past few hours we have received reports of people who have been killed and dozens of people who have been injured and detained.” The NGO Foro Penal said that six people have died in the clashes. UN human rights chief Volker Türk said he was “extremely concerned” about the the situation, adding: “I am alarmed by reports of disproportionate use of force by law enforcement officials along with violence by armed individuals supporting the government.” He said: “Those responsible for human rights violations must be held to account.” Opposition supporters have urged the armed forces to take their side. But on Tuesday defense minister Vladimir Padrino López issued a statement reiterating the armed forces’s “most absolute loyalty and unconditional support” for Maduro. He accused far-right extremists of committing “acts of terror” in a bid to overturn Maduro’s legitimate victory. Padrino López said the military was committed to peace. “But at the same time we will take forceful action … to maintain internal order,” he added ominously. As the crisis intensified, Venezuela’s diplomatic isolation grew, although crucially the governments of Russia and China have recognized Maduro’s victory. Their support has been critical to Maduro’s ability to overcome previous challenges, such as the failed 2019 bid to topple him by the now exiled opposition leader Juan Guaidó. On Monday Venezuela’s foreign minister, Yván Gil, said scores of diplomats from seven Latin American countries who have criticsed the election result would be expelled. Flights to the Dominican Republic and Panama were halted. Brazil’s leftwing government – which has long-standing ties to the Chavista movement dating back to President Lula’s first 2003-2011 presidency, when Maduro’s mentor Hugo Chávez was in power – has refrained from directly challenging the result. But, tellingly, Brasília has publicly urged Maduro to publish election data that it believed was essential “to the transparency, credibility and legitimacy” of the vote. Amorim said Maduro had assured him such data would be published in the next two days during their encounter at the presidential palace on Monday. “I thought he seemed calm. I didn’t think he seemed to be in a fearful or bellicose mood,” Amorim said of his host. Amorim said he had also met González during his time in Caracas, who he described as having a “pleasant, peaceful temperament”. “But there are, as you know, other forces at play that are more bellicose,” he added of the opposition. The veteran diplomat, who is Brazil’s former foreign and defense minister, said he hoped Colombia, Mexico and Brazil could help defuse the situation by encouraging dialogue. “Improper external interference,” would not help. Lula and the US president Joe Biden were expected to discuss the crisis on Tuesday afternoon, at the request of the White House.