Manuel Valls, 54, stunned his Socialist colleagues by saying that he wanted to stand in June's parliamentary elections as a candidate on Mr Macron's side.
Prime Minister Modi had earlier greeted Emmanuel Macron for his "emphatic victory" in the French Presidential election.
There is scepticism however about Macron's ability to win a majority with En Marche candidates, meaning he might have to form a coalition.
On May 7, Macron was elected French president, taking around 65 percent of the vote to Le Pen's 35 percent.
Emmanuel Macron won a resounding victory in the French presidential election but the focus shifted immediately Monday to whether he can govern the country without the support of a traditional party.
The France Insoumise (France Unbowed) of firebrand leftist Jean-Luc Melenchon was trailing in fourth with 13-15 percent, ahead of the tattered Socialists of outgoing President Francois Hollande on eight-nine percent.
On Monday, though, Hollande gripped Macron's arm before the two men walked side by side and then announced the transfer of power would take place on Sunday, a week after the election. The ceremony marked decades of peace in Western Europe, something Macron made a cornerstone of his campaign against Le Pen's brand of nationalist populism.
At 39, the pro-EU former investment banker will become France's youngest-ever president when he is inaugurated next weekend after crushing far-right leader Marine Le Pen in a run-off vote on Sunday. Le Pen had called for France to leave the 28-nation European Union and drop the shared euro currency in favor of bringing back the French franc.
Sylvie Goulard, a French deputy to the European Parliament, said Mr Macron would make Berlin his first official visit, with perhaps a stop to see French troops stationed overseas as well.
After winning the first round two weeks ago, Macron had been accused of behaving as if he was already president; on Sunday night, with victory finally sealed, he was much more solemn. Ferrand also said the names of Macron's 577 candidates in the legislative elections would be announced this Thursday. In interviews Monday, National Front officials said the party founded by her father would get a new name to try and draw in a broader spectrum of supporters.
"The only credible change would be for Marion Le Pen to replace Marine Le Pen, because she is clearly superior to her", di Borgo said.
Republicans vice-president Laurent Wauquiez emphasised Macron's vulnerability, noting he had been a grudging choice of president for many voters, who backed him only to block Le Pen.