London Mayor Sadiq Khan says Donald Trump's electon shows progress "is not inevitable" in the battles with racism, climate change and for women's rights.
Mr Khan, who had a long-running row with Trump during his first term in the White House, said that many around the world will be feeling "fearful". The Republican, who is a convicted felon, declated victory in the early hours of the morning.
Meanwhile Lib Dem leader Sir Ed Davey said it is a "dark, dark day" and branded Trump a "dangerous, destructive demagogue". Tens of thousands turned out to protest in London on Trump's last official visit to the UK, with a huge blimp depicting him as a giant baby flown above the capital.
The 78-year-old is likely to face similar scenes next time he comes. Mr Khan said: "I know that many Londoners will be anxious about the outcome of the US presidential election.
"Many will be fearful about what it will mean for democracy and for women's rights, or how the result impacts the situation in the Middle East or the fate of Ukraine. Others will be worried about the future of Nato or tackling the climate crisis."
Mr Khan went on: "The lesson of today is that progress is not inevitable.
"But asserting our progressive values is more important than ever - re-committing to building a world where racism and hatred is rejected, the fundamental rights of women and girls are upheld, and where we continue to tackle the crisis of climate change head on."
There has long been bad blood between the Labour London Mayor and Trump. Mr Khan criticised his attitudes toward Muslims and called for his visit to the UK to be cancelled.
The then-US President - who also made misleading claims about Mr Khan's response to the London Bridge terror attack -described the Mayor as “a stone cold loser” and “very dumb”.
Following Trump's victory declaration, Lib Dem leader Sir Ed hit out. He said: "This is a dark, dark day for people around the globe. The world’s largest economy and most powerful military will be led by a dangerous, destructive demagogue.
"The next President of the United States is a man who actively undermines the rule of law, human rights, international trade, climate action and global security. Millions of Americans – especially women and minorities – will be incredibly fearful about what comes next. We stand with them.
"Families across the UK will also be worrying about the damage Trump will do to our economy and our national security, given his record of starting trade wars, undermining NATO and emboldening tyrants like Putin."
The remarks stand in stark contrast from warm words by PM Keir Starmer and Foreign Secretary David Lammy. Mr Starmer said: “Congratulations President-elect Trump on your historic election victory. I look forward to working with you in the years ahead. As the closest of allies, we stand shoulder to shoulder in defence of our shared values of freedom, democracy and enterprise.
“From growth and security to innovation and tech, I know that the UK-US special relationship will continue to prosper on both sides of the Atlantic for years to come.”
Mr Lammy said: "Congratulations to @realDonaldTrump on your victory. The UK has no greater friend than the US, with the special relationship being cherished on both sides of the Atlantic for more than 80 years. We look forward to working with you and @JDVance in the years ahead."