Meryl Streep picked up the third Oscar of her career on a night when Hollywood made a big noise about a silent film.
The Artist - a tribute to the world of black and white silent cinema that disappeared with the arrival of sound - won five awards including best picture, best actor and director.
Its director, Michel Hazanavicius, paid tribute to his wife - and one of the film's stars - Berenice Bejo who lost out on the supporting actress award to Octavia Spencer.
Speaking after the film's cast and crew went up to accept the best picture award, he told Bejo: "You inspired the movie and you're the soul of the movie and the positive feeling of the movie. Thank you for being in the movie and in my life".
The French director also had a message to his children, telling them: "It's six in the morning in Paris. Go to bed."
He paid tribute to his own cinematic hero, saying: "I want to thank three persons, I want to thank Billy Wilder, I want to thank Billy Wilder and I want to thank Billy Wilder".
Earlier he had picked up the best director award from Fatal Attraction star Michael Douglas and thanked the film's cast including Uggie the Dog, adding: "I think he doesn't care, I'm not sure he understands me."
The best actress Oscar was presented by Colin Firth to Streep whom he described as "unreasonably good" in her role as former prime minister Margaret Thatcher in The Iron Lady.
The actress joked she could hear "half of America going 'Oh no"' when her name was read out before thanking her husband, Don Gummer, and also her long-time colleague, J Roy Helland, who picked up the make-up award earlier in the night for his work on the film.
She said: "I really want to thank all my colleagues, all my friends, I look out here and see my life before my eyes, my old friends, my new friends and really this is such a great honour but the thing that counts the most with me is the friendships and the love and the sheer joy we have shared making movies together."
Two other popular winners were Christopher Plummer and Octavia Spencer who picked up the supporting actor and actress gongs respectively.
Plummer, who at 82 is the oldest actor ever to win an Oscar, held up his statuette and said: "You're only two years older than me darling, where have you been all my life?"
He paid tribute to his co-star in Beginners, Ewan McGregor, whom he said he would "happily share this award with, if I had any decency, but I don't".
Speaking backstage, he said: "Well, it is sort of a renewal, it's not a beginning exactly, but it has recharged me and I hope I can do it for another 10 years at least. I'm going to drop dead wherever I am, on stage or on the set. We don't retire in our profession, thank God."
A clearly emotional Spencer accepted her award from Welsh-born actor Christian Bale for her role in The Help.
The tearful actress, who was given a standing ovation, thanked the academy for "putting me with the hottest guy in the room" before thanking her family in Los Angeles and Alabama.
Fighting back the tears, she ended by saying: "Please wrap up, I'm wrapping up, I'm sorry I'm freaking out. Thank you world."
Natalie Portman presented the best actor award to Jean Dujardin for The Artist.
The French star told the audience "I love your country" and thanked swashbuckling screen idol Douglas Fairbanks Junior for inspiring his role as the silent film hero.
The Artist also picked up awards for costume design and original score.
Another ode to old Hollywood, Martin Scorsese's 3D Hugo also picked up five awards but in technical categories including cinematography and art direction.
The award for adapted screenplay went to the George Clooney film, The Descendants, while Woody Allen won the original screenplay Oscar for Midnight in Paris - his tribute to the French capital in the 1920s when it was home to expat-American writers including F Scott Fitzgerald and Ernest Hemingway.
The veteran star famously rarely attends the Oscars so Angelina Jolie accepted it on behalf of the academy.
The six stars of Bridesmaids handed out prizes for live action short film to Terry and Oorlagh George's Northern Ireland-set The Shore.
Mr George dedicated his win to "the people of Northern Ireland, Protestant and Catholic, who after 30 years of war sat down, negotiated a peace and proved to the world that the Irish are great talkers".
Speaking backstage, Mr George, who wrote the Guildford Four film In The Name of the Father, said he would celebrate by going back "to the little village where we shot this".
He said: "Already tonight they had an Oscar party at a place called The Anchor Bar, so I'm going to return with the prize, and then hopefully use it to promote, not just the peace process in northern Ireland, but tourism and everything that's going on there. So I hope that this is just a reaffirmation that things have changed there and that we're trying to move on and it's a great place to be."
The award for foreign-language film was presented by Sandra Bullock to the Iranian film A Separation, while best documentary feature was won by Undefeated - about an American football team.
Chris Rock gave the award for animated feature film to Rango made by Pirates of the Caribbean director Gore Verbinski.
Will Ferrell and Zach Galifianakis presented the award for original song to Flight of the Conchords star Bret McKenzie for his song Man or Muppet from the latest Muppets film.
He thanked his parents for "never telling me to get a real job".
The award for documentary (short subject) went to Saving Face - about a London-based plastic surgeon who treats Pakistani women maimed in acid attacks.
The Oscar for animated short film went to The Fantastic Flying Books Of Mr Morris Lessmore.
The audience also saw a tribute to some of the names that have died in the last 12 months including Ken Russell, Whitney Houston and Elizabeth Taylor to the strains of What a Wonderful World performed by Esperanza Spalding.
The show was opened by Morgan Freeman before host Billy Crystal appeared in a spoof showreel based on scenes from nominated films including The Descendants, The Help and The Artist.
Crystal told the audience it was his ninth time hosting the event, joking "so tonight, just call me War Horse".
He then launched into a song based on the films nominated for the best picture Oscar, after joking "Nothing can take the sting out of the world's economic problems like millionaires presenting each other golden statues".
Earlier in the night, stars including Gwyneth Paltrow, Brad Pitt, Clooney, Jessica Chastain and Bejo, greeted fans, signed autographs and posed for photographers outside the Kodak Theatre in Los Angeles.
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WINNERS AT 84TH ACADEMY AWARDS
Here is the full list of winners for the 84th Academy Awards:
Jean Dujardin - The Artist
Meryl Streep - The Iron Lady
Christopher Plummer - Beginners
Octavia Spencer - The Help
Midnight In Paris
The Artist - Michel Hazanavicius
Animated feature film:
Documentary (short subject):
The Girl With The Dragon Tattoo
Foreign language film
A Separation (Iran)
The Iron Lady
Music (original score)
The Artist - Ludovic Bource
Music (Original Song)
Man Or Muppet (The Muppets)
Short film (animated):
The Fantastic Flying Books Of Mr Morris Lessmore
Short film (live action):