Socialist candidate Benoît Hamon, who received a humiliating 6.35 percent of the vote, called for a vote for Macron when he accepted defeat on Sunday night.
But he added that the former economy minister "is not part of the left and has no mandate to represent it".
At least six Socialist ministers called for support for their former colleague with varying degrees of enthusiasm.
Some leading Socialists, including former prime minister Manuel Valls, backed Macron against their own candidate in the first round.
Valls aly Jean-Marie Le Guen on Monday said he hoped that the "responsible, governmental left" would be part of a Macron government.
Macron's En Marche! (Let's Go!) movement was founded during the presidential campaign and it is unclear how many seats it can win in the parliamentary election that will follow the presidential poll.
Mainstream right against Le Pen
Commenting that "abstention is not in my genes", Fillon himself said he would vote Macron on Sunday night.
Former prime minister Alain Juppé, whom Fillon defeated in the right-wing primary, tweeted that he supported Macron "without hesitation" and called on voters to do the same.
Jean-François Copé, who is to the right of Juppé and has previously refused to back Socialists against the National Front, also said he would vote Macron although he would be "sick at heart" in doing so, a sentiment echoed by the party's parliamentary leader Christian Jacob, while declaring himself "a determined opponent of the left and M Macron is the left".
Further to the right again, Rhône-Alpes regional president Laurent Wauquiez simply called on voters not to cast their ballots for Le Pen, and Eric Ciotti, an MP from the south of France, refused to "enter into partisan deals".
Mélenchon under fire
Meanwhile hard-left candidate Jean-Luc Mélenchon, who came in fourth with 19.64 percent, came under fire from Socialists and Macron supporters for refusing to express a preference on Sunday evening.
Commenting that the "mediacrats and oligarchs" were delighted by the result, he said both second-round candidates "want to prolong the lives of the present institutions".
Mélenchon said he would consult the 400,000 people who backed his candidacy on the internet on what line to take, with a choice of voting Macron, abstaining or casting a blank vote.
Communist Party national secretary Pierre Laurent, who supported Mélenchon, called on voters to cast "the other ballot" to beat Le Pen.
Socialist Party first secretary Jean-Christophe Cambadélis and government spokesperson Stéphane Le Foll slammed Mélenchon's statement, Le Foll accusing him of "showing his disappointment rather than speaking of France".
To read our coverage of France's 2017 presidential election click here