Flags were lowered to half-mast at Britain's parliament to mark the death of Margaret Thatcher on Monday, and flowers piled up at her London home -- but at the other end of the political spectrum, left-wingers joyfully planned parties to celebrate her departure.

On the streets of London's financial district -- whose power was fuelled by Thatcher's deregulation of the financial sector -- many passers-by reacted with dismay to the passing of the former Conservative premier.

"It's a shame, a crying shame. She's a good woman," said Alan Whiteford, a law firm employee.

"It's a sad day," added banker Nick Daking.

At Thatcher's former home in central London, a pile of flowers was growing on the doorstep.

"The greatest British leader and a true lady," one card read. "You make Britain what it is."

But in the edgy south London neighbourhood of Brixton, sworn enemies of the former Iron Lady were planning a huge street party for Monday evening -- with more than 600 people listed as attending on Facebook.

Coal miners were among Thatcher's bitterest foes during her 1979-90 premiership -- and for one senior mining official marking his birthday on Monday, her death was the icing on the cake.

"I'm having a drink to it right now," David Hopper, regional secretary of the National Union of Mineworkers (NUM) in northeast England, told AFP with unabashed glee.

"It's a marvellous day. I'm absolutely delighted. It's my 70th birthday today and it's one of the best I've had in my life."

Others on the left also hailed Thatcher's departure as a cause for celebration.

"We'll be glad to see the back of her," Judith Orr, editor of the far-left Socialist Worker weekly newspaper, told AFP.

"She ruined the lives of tens of millions of working class people in Britain.

"And she rejoiced in war. She was the one who said we should rejoice in the sinking of the Belgrano, in the deaths of hundreds of young Argentine conscripts.

"That was one of her most disgusting moments, but there is a long list of crimes."

Rights activist Peter Tatchell described the former Iron Lady as "extraordinary but heartless", saying she had presided over the decimation of Britain's manufacturing base and introduced "Britain's first new anti-gay law in 100 years", Section 28.

Yet he conceded that as Britain's first and still only woman prime minister, she had achieved something significant.

"To her credit, she shattered the sexist glass ceiling in politics and got to the top in a man's world."

A single daffodil was placed at feet of a statue of Thatcher outside the chamber of the House of Commons, or lower house of parliament, with a card saying: 'You were an inspiration to women'.

Online reaction to Thatcher's death was just as mixed.

"A great lady," wrote technology tycoon Alan Sugar. "She changed the face of British politics, created opportunity for anyone to succeed in the UK. RIP."

Even One Direction singer Harry Styles paid tribute, tweeting "RIP Baroness Thatcher" -- but the boy band singer's message left many young fans, born after her era, confused as to who he was talking about.

Others did not hold back in condemning Thatcher, just hours after her death.

"May she burn in the hellfires," tweeted left-wing firebrand lawmaker George Galloway.

"I wish there was a Margaret Thatcher statue in Glasgow," posted Scottish comedian Robert Florence. "I'd like to be seen on the news toppling it and hitting it with my shoes."

Many of those celebrating her passing marked their tweets with the hashtag #nowthatchersdead -- causing panic among fans of the US singer Cher, as some took this to read: "Now that Cher's dead".