Led by women in pink "pussyhats," hundreds of thousands of people flooded the streets of Washington and cities across the United States in a massive outpouring of defiant opposition to President Donald Trump. Roused by fiery speeches, the protesters sent out a resounding message of resistance the day after the Republican hardliner took office with a vow to roll back the legacy of his predecessor Barack Obama. "I know that we can do better, we have to fight for the change we want to see," said Michelle Phillips, a 45-year-old recent American citizen, who said she came to take a stand against Trump's "platform of hate and bigotry." A sea of women and men -- teens, pensioners, parents with toddlers on their shoulders -- swarmed up the streets around the White House in a good-natured but determined show of unity. "Women won't back down," "Women's rights are human rights" and "Thank you Trump -- you turned me into an activist," read some of the hundreds of handmade signs held aloft in the capital. Organizers estimated the turnout for the "Women's March on Washington" at half a million, double initial expectations, with huge crowds reported at sister marches nationwide, from Chicago to New York, Boston and Los Angeles. Saturday's rallying cry was heard far beyond America's shores, with organizers saying over 2.5 million people signed up to take part in one of more than 600 marches being held worldwide. One of the largest was in London, where tens of thousands of women, men and children marched chanting "Dump Trump." The human tide flooding the US capital appeared to dwarf the throngs of Trump supporters in red "Make America Great Again!" caps who the day before had cheered his swearing-in. Washington's Metro stations were overwhelmed as trains packed to bursting ferried cheering, clapping marchers into the city -- many wearing knitted "pink pussyhats" in an allusion to Trump's videotaped boasts of grabbing women's "pussies" with impunity. By 11 am, the city's Metrorail system said it had moved 275,000 people, eight times a typical Saturday. Trump's defeated rival Hillary Clinton tweeted her support to the protesters, while former secretary of state John Kerry was spotted in the crowd -- a day after leaving office -- with his dog on a pink leash. And Pop diva Madonna, wearing a black pussyhat of her own, made an impromptu appearance on the protest's main stage near Washington's National Mall to deliver an expletive-laden indictment of the president. "Welcome to the revolution of love," the 58-year-old intoned. "To the rebellion. To our refusal as women to accept this new age of tyranny." - Trump's first full day - For his first full day in the world's most powerful office, Trump attended a multi-faith service at Washington National Cathedral before visiting the headquarters of the CIA, an agency he feuded with bitterly before taking office. "I am with you 1,000 percent," Trump said in a short address to CIA staff -- during which he also complained about media coverage of his inauguration which he said played down the turnout. Trump's inaugural speech on Friday set the tone for his presidency: proudly populist, fiercely nationalist and determined to break with Obama's legacy. His first act in office -- signing an executive order aimed at freezing Obama's signature health care law -- was a potent gesture in that direction, with more such actions expected to follow. But if Friday was Trump's day -- marred by sporadic outbreaks of vandalism and more than 200 arrests -- Saturday belonged to demonstrators with fresh memories of his fat-shaming a former beauty queen, sex assault allegations and a controversial stance on abortion. Filmmaker Michael Moore, a march organizer, noted that his copy of the Washington Post was bannered with the headline "Trump Takes Power." "I don't think so. Here is the power," he said, gesturing to the crowd. - 'Railroad' - Jennifer Behr, a 42-year-old accessory designer, rode a packed train from Baltimore to make her voice heard. "It's important we assert our majority and we have a large physical presence to show Trump and the Republicans that they cannot railroad our country," she said. While Trump won 42 percent of the women's vote, millions who did not vote for him worry that gender rights and other progress on women's health, contraception and abortion could be chipped away. The Women's March began with a simple Facebook post from Hawaii grandmother and retired lawyer Teresa Shook to about 40 of her friends -- but word travelled quickly and the event took on a life of its own. Dozens of progressive groups backed the march, as well as Amnesty International and Planned Parenthood, the women's health care provider that is a Republican target because of the abortion services it provides.