With BJP headed for a big majority in the Lok Sabha, Modi is set to succeed Manmohan Singh as the country's 14th prime minister.
"Prime Minister Manmohan Singh calls Narendra Modi and congratulated him on his party's victory in the Lok Sabha elections," according to a message on the twitter handle of the Prime Minister's Office.
On Saturday, Manmohan Singh will address the nation and then will submit his resignation to President Pranab Mukherjee.