France's ambassador to Britain has thanked the Met as tens of thousands of French voters in the UK cast their ballots for the decisive presidential run-off.
Sylvie Bermann said they had "worked closely" with the force as several armed officers patrolled outside the Lycee Charles de Gaulle polling centre in Kensington, south London.
Some two million French people live abroad, with an estimated 350,000 in the UK, the embassy said.
Of the 250,000 said to be living in London, almost 100,000 are registered voters.
Ms Bermann said: "I hope the turnout will be good, it's two million people, so it counts, I think it's important.
"We've worked closely with the Met and thank them for what they did to ensure our security for the first round and this time also."
As of midday (11am BST), turnout was 28.2%, which is about two points lower than at the same time in 2012.
The French Interior Ministry will update turnout figures at 5pm (4pm BST).
By the end of the day, France will know its next president - centrist Emmanuel Macron or former National Front leader Marine Le Pen.
Voter Thibaud Marec told the Press Association: "Voting is the most important tool we have as a citizen in a democratic country to express what we feel.
"It's important not to miss this opportunity despite all the frustrations in the country at the moment.
"It's very tense ... two parts of the country are definitely split apart in different directions. I hope the outcome of the election tonight won't be too much of a trouble-generator."
Polls close at 7pm (6pm BST) in the UK, though remain open until 8pm (7pm BST) in some large cities.
As soon as polls close at 8pm (7pm BST), exit results will be announced on French television.
The results should give a fairly accurate idea of who has won - two weeks ago they predicted Ms Le Pen and Mr Macron would get to the second round.
Former Ukip leader Nigel Farage gave his backing to Ms Le Pen, saying she is "the real deal", but conceded she was more likely to become president in 2022 than this election.
"She is a proper, genuine Eurosceptic and under her the French National Front is not about race but it is about sovereignty," he told ITV's Peston on Sunday.
She would be "better for France than Macron" and "better for Brexit Britain".