Prime Minister Modi, who is the chief guest at the event, lit lamps - a traditional practice. Sri Lankan President Maithripala Srisena was also present on the occasion.
Several Buddhist monks chanted prayers during which Mr Modi closed his eyes and kept his hands folded.
Speaking at the event, Mr Wickremesinghe thanked PM Modi for being the chief guest at the event.
"Proud to host Vesak Day celebrations in Colombo. I want to thank Prime Minister Modi for being the chief guest during the celebrations," Mr Wickremesinghe said.
Later today, PM Modi will inaugurate a hospital built with India's assistance of Rs 150 crore.
He will also address Indian-origin Tamils against the backdrop of India's insistence that Sri Lanka should undertake devolution of power to the ethnic community as part of the reconciliation process.
Prime Minister Narendra Modi arrived in Colombo yesterday on a two-day visit which is mainly aimed at reinforcing the traditional connect between India and Sri Lanka at a time when China is seeking to make inroads in the island nation.