French presidential hopeful Emmanuel Macron has been heckled by factory workers in Amiens after a visit by his far-right rival, Marine Le Pen.
Ms Le Pen upstaged her centrist rival earlier by turning up to speak to the workers as he met their union representatives a few miles away.
Mr Macron, who is far ahead in opinion polls, is in Amiens, his home town, amid accusations of complacency.
He got another boost when former President Nicolas Sarkozy endorsed him.
François Fillon, the candidate of Mr Sarkozy's own, centre-right Republican party, was knocked out in the first round, leaving uncertainty over how party supporters would vote in the second.
Opinion polls taken since the first round on Sunday suggest Mr Macron, candidate of the En Marche (On The Move) movement, will easily beat Ms Le Pen, who has temporarily stood down as leader of the National Front.
The polling average line looks at the five most recent national polls and takes the median value, ie, the value between the two figures that are higher and two figures that are lower.
Le Pen takes the initiative: By Hugh Schofield, BBC News, Paris
This was a neat publicity coup for Marine Le Pen. As Emmanuel Macron was holding talks at the local chamber of commerce, the National Front leader made an unannounced visit to the picket line at the Whirlpool factory where she was greeted with enthusiasm by many of the striking workers.
She told reporters there that Mr Macron's decision to meet union officials in the comfortable surroundings of an office - rather than come to the factory gates as she had - was a sign of someone who deep down was contemptuous of working people's lives.
Mr Macron retorted that Marine Le Pen was cynically exploiting the situation for political ends and that it was more important to work out a long-term solution for the factory than to score cheap points in front of the cameras.
The clash is symptomatic of a second-round campaign in which so far Marine Le Pen has clearly had the initiative. Some in Mr Macron's camp are urging him to up his game and not assume that his strong lead in the polls will necessarily endure.
The Whirlpool household appliance factory in Amiens, in the rustbelt of France's industrial north, faces the threat of closure with outsourcing to Poland.
Mr Macron earlier saw union delegates at the local chamber of commerce, with TV pictures showing them in discussion in a grey meeting room. Meanwhile, Ms Le Pen was having selfies taken outside the actual factory a few miles away.
When he visited the actual factory, surrounded by journalists, he was booed and whistled.
"There is no work!" a woman shouted repeatedly as he sought to address the workers.
"It's important not to feed anger but to be up to meeting expectations," he told the crowd. "Of course there's anger in the country, there's anguish, there's a responsibility to take, that's why I'm here."
Ms Le Pen, whose new campaign slogan is "Choose France", tweeted (in French) photos of her meeting the workers and said: "With me, their factory won't close!"
She seeks to portray her pro-EU opponent as the candidate of "runaway globalisation", hoping to pick up votes from the extreme left despite her own far-right background.
Leading members of the ruling Socialist Party have criticised Mr Macron, a former Socialist minister, for not fighting hard enough in the run-off campaign.
"He was smug," Socialist Party boss Jean-Christophe Cambadelis told French radio. "He wrongly thought that it was a done deal. It's not a done deal."
In a statement (in French) on Facebook, Mr Sarkozy said he would vote for Mr Macron and was retiring from politics himself.
"I consider that the election of Marine Le Pen and the launch of her project will bring serious consequences to our country and to the French," he wrote.
"I will therefore be voting in the second round of the presidential election for Emmanuel Macron. It's a choice of responsibility, which is not in any case a support for his project."