It came as former president Nicolas Sarkozy became one of the most high-profile figures to urge voters to put aside party differences and back the centrist Mr Macron over the far-right Ms Le Pen.
The two final-round opponents have begun a two-week sprint to gather votes from the 17 million people who backed other candidates in the first round, with 7.7 million of those supporting hardline left-winger Jean-Luc Melenchon.
Ms Le Pen’s camp wants to seduce those voters by opposing the president’s plans to raise the state pension age from 62 to 65 to increase productivity and cut the cost of France’s expensive welfare state.
The president has been promising pension reform since his first campaign in 2017, but his proposals sparked protests from the Yellow Vest movement and were eventually shelved during the coronavirus pandemic.
Although the move to 65 is in Mr Macron’s manifesto for the next five years, he said on his first day of campaigning for the April 24 second round he was willing to push it back until 2030 if “people are too anxious” to do it sooner.
Lowering the threshold to 64 is not out of the question because 65 “is not a dogma”, he said as he answered questions in one of Ms Le Pen’s northern heartlands.
Finance Minister Bruno Le Maire, a supporter of Mr Macron, separately said the move to 65 would not be scrapped but “there will be options for discussing details”.
The softer line was taken by rival camps as an attempt to persuade left-wing voters to go the polls, after Mr Melenchon told his supporters not to give “one single vote” to the far right but stopped short of endorsing Mr Macron.
“It's a manoeuvre by Emmanuel Macron to try to win over, or at least to mitigate the opposition of the left-leaning voters,” Ms Le Pen told France Inter radio on Tuesday.
Manuel Bompard, the head of the Melenchon campaign, suggested a referendum on pensions as he voiced dissatisfaction with both remaining candidates’ stances on the issue.
Mr Melenchon supports reducing the pension age to 60, an idea Ms Le Pen once supported but which she has largely backed away from in favour of keeping it at 62.
“I tell Macron this: If he really wants to appeal to our voters... he has to make a clear commitment,” said Mr Bompard.
The president’s camp has sought to turn public attention to Ms Le Pen’s hard-line views and a record of Kremlin-friendly statements that have come back into the limelight since Russia invaded Ukraine.
Mr Sarkozy, the president from 2007 to 2012 whose centre-right Republicans were eliminated in Sunday’s first round, cited the war in Ukraine as part of his reasoning for supporting Mr Macron this time.
“I believe he has the necessary experience faced with a grave international crisis… and his commitment to Europe is clear and unambiguous,” said Mr Sarkozy.
He urged voters to show “republican values” by uniting against Ms Le Pen as they have done in the past, amid concerns that this habitual dam-building against the far right is no longer a certainty.
Mr Macron won 28 per cent of votes in the first round. Ms Le Pen won 23 per cent, with Mr Melenchon scoring a higher-than-expected 22 per cent.
Although nearly all second-round polls have shown Mr Macron in the lead, some have him only a few points ahead and all suggest a far narrower race than when the same two candidates faced off in 2017.
Mr Macron won by a landslide that year after his new centrist party upended French politics, leaving the mainstream left and right struggling for survival.
Valerie Pecresse, the defeated candidate from Mr Sarkozy’s party, on Monday issued an appeal for donations after failing to reach the 5 per cent threshold necessary for campaign debts to be reimbursed by the state.