Heavy rush was witnessed at temples, including at the famous hill shrine of Lord Ayappa at Sabarimala, Sree Krishna Temple at Guruvayur and Sree Padmanabha Swamy Temple in Thiruvananthapuram.
Onam is celebrated to honour King Mahabali, who according to legend ruled the state once, under whom the people were equal, prosperous and happy.
People believe that on this particular day of Thiruvonam Lord Mahavishnu in his fifth avatar as Vamana, appeared in the kingdom of King Mahabali and sent him to the netherworld.
Legend has it that it was on this particular day the spirit of King Mahabali visits the people of Kerala.
People thronged markets till last night to buy their favourite items for Onam celebrations.
The day also witnessed a protest in front of the secretariat here by Kerala State road transport corporation employees who staged a hunger strike demanding payment of salary and pension on time.
The week-long tourism celebrations got off to a colourful start last evening with a music and dance performance by noted actress Manju Warrier.