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Little Mix and Taylor Swift were among the big winners at the ceremony, while Sam Smith, The Saturdays, Foxes and Ella Henderson were also in attendance.
Leigh-Anne Pinnock, Jesy Nelson, Perrie Edwards and Jade Thirlwall won Best Band at the event.
Lambert presented the group with their Best Band prize.
The singer turned to reveal a loose-knotted train on her outfit.
The ceremony took place in London's Berkley Square Gardens.
Emma won the gong for Best Presenter.