The Artist triumphs at the Oscars
Silent movie The Artist has triumphed at the Oscars, winning five awards including best picture, best director and best actor for Jean Dujardin.
Director Michel Hazanavicius - winning on his first ever nomination - thanked the dog, Uggie, who appears in the film but added: "I don't think he cares."
Dujardin said of his character: "If George Valentin could speak, he would say 'Wow! Victorie! Genial! Merci!'"
The film also won the Oscars for best original score and best costumes.
Martin Scorsese's Hugo also won five Oscars, mainly in technical categories.
Meryl Streep won best actress for her portrayal of former British Prime Minister Margaret Thatcher in The Iron Lady - her 17th Oscar nomination and third Oscar win.
She thanked the Academy "for this inexplicably wonderful career".
"When they called my name I had this feeling I could hear half of America going: 'Aww no. Not her again'. But, you know, whatever.
"I look out here and I see my life before my eyes. My old friends, my new friends. This is such a great honour but the thing that counts the most for me is the friendships… Thank you. All of you, departed and here," she added.
Dujardin broke into his native French language in celebration shouting: "Wow, victory!"
"Thank you to the Academy. It's funny because in 1929, it wasn't Billy Crystal but Douglas Fairbanks who hosted the first Oscars ceremony. Tickets cost $5 and it lasted 15 minutes. Times have changed."
1929 was the last year that a silent movie won an Oscar.
Canadian actor Christopher Plummer became the oldest Oscar winner at 82 by taking the best supporting actor prize.
He was widely tipped to win for his portrayal of a father who comes out as a gay man after his wife dies in Beginners.
Plummer thanked his real-life wife who, he said, deserved "the Nobel Peace Prize for coming to my rescue every day".
The Help's Octavia Spencer won the best supporting actress Oscar and gave an emotional acceptance speech, receiving a standing ovation from the audience.
"Thank you Steven Spielberg for changing my life...oh my God, thank you... I'm freaking out," she told the audience, after struggling up to the stage in a floor-length gown.
Best adapted screenplay went to Alexander Payne, Nat Faxon and Jim Rash for The Descendants, starring George Clooney.
Veteran screenwriter and director Woody Allen won best original screenplay for Midnight in Paris but was not there to collect the award.
The first two awards of the night went to Hugo for cinematography and art direction.
Robert Richardson was cinematographer on Martin Scorsese's 3D film and Francesca Lo Schiavo was art director.
And later, the film about an orphan who lives in a train station picked up a further three Oscars, all in technical categories.
Best sound editing was won by Philip Stockton and Eugene Gearty.
Hugo's Tom Fleishman and John Midgley won the Oscar for sound mixing and the film also picked up the award for best visual effects.
Rango won best animation, a first Academy award and nomination for director Gore Verbinski, who said it was "made by grown-ups acting like a bunch of children".
The film features the voice of Johnny Depp, who plays a chameleon.
Best animated short film was The Fantastic Flying Books of Mr Morris Lessmore.
The Oscar for costume design went to Mark Bridges for The Artist, who thanked the Academy "for making a lifelong dream come true".
The best make-up prize went to J Roy Helland and British artist Mark Coulier for The Iron Lady.
Iran's A Separation became the first Iranian film to win an Oscar when Sandra Bullock presented director Asghar Farhadi with best foreign language film.
Set in contemporary Iran, it tells the story of a marriage break-down.
Best film editing went to Kirk Baxter and Angus Wall for Girl with the Dragon Tattoo - the pair also won last year for The Social Network. Both films were directed by David Fincher.
The Oscar for best original song was won by Bret Mackenzie for Man or Muppet from the soundtrack to The Muppets.
Best documentary went to Undefeated, a film about an inner city American football team whose fortunes are turned around by a new coach.
The executive producer of the film was rapper Sean 'P Diddy' Combs.
Northern Ireland film The Shore won the best live action short film.
Saving Face, about a British-Pakistani doctor who helps women who have been injured in acid attacks, won best documentary short.
Earlier, Morgan Freeman introduced the evening before a comic video was shown of George Clooney waking up host Billy Crystal with a kiss - in a parody of his nominated film The Descendants.
Freeman said: "All of us are mesmerised by the magic of the movies. This magnificent event allows us to celebrate the present and look back at its magnificent past."
Crystal hosted the 84th Oscars ceremony at the Kodak theatre in Los Angeles.
He joked: "This is my ninth time - just call me War Horse."
On the red carpet, British comedy actor Sacha Baron Cohen turned up dressed in a white military uniform and sporting a beard and sunglasses, promoting his upcoming film The Dictator.
Cohen arrived holding an urn he jokingly claimed contained the ashes of Kim Jong Il, the late leader of North Korea.
Cohen then tipped the container on to American Idol host Ryan Seacrest.