The festival is celebrated to honour King Mahabali, who according to legend ruled the state once, under whom the people were equal, prosperous and happy.
Bachchan, 72, took to the micro-blogging site and wrote, "T 1976 - A very happy Onam to all... Festival predominantly celebrated with gusto in Kerala and Malayalis..."
"Dream Girl" Malini tweeted, "Wish you all a wonderful Onam! Have a great day!"
"Wishing all of you a very Happy Onam. May this day bring prosperity, happiness and peace in your lives," said Sridevi Boney Kapoor.
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National-award winning director, Bhandarkar, tweeted, "May the colour and lights of Onam fill your home with happiness and joy. Have the most beautiful Onam."
"Kolaveri Di" star Dhanush wrote, "Happy onam."
Tamil film actor Suriya, who was seen in the Hindi movie "Rakta Charitra", said, "Happy Onam to all my dearest Kerala friends! Wishing you n all at home Thiruonam Ashamshakal...! All happiness...