Emmanuel Macron and other G20 leaders laughed at Boris Johnson for arriving late for the G20 summit photoshoot.
The British prime minister and Canadian President Justin Trudeau were delayed for the traditional “family photo” on Saturday morning and received humorous chiding sounds from other world leaders.
A video clip showed Macron smiling at Johnson when he arrived before fist-bumping him amid a row over fishing rights between the UK and France.
France is threatening to block British boats from its ports and tighten checks on vessels if an issue over a lack of licences for small French vessels to fish in British waters is not resolved by Tuesday.
French authorities seized a British trawler as tensions intensified.
Johnson has refused to rule out triggering the dispute mechanism clause in the Brexit trade agreement after suggesting France had breached the terms of the deal with its fishing threats.
Watch: G20 world leaders arrive in Rome for summit
Meanwhile, the heads of the world's 20 biggest economies kicked off two days of talks on Saturday for the first face-to-face G20 summit in two years.
Italian Prime Minister Mario Draghi greeted leaders from various countries, including Macron, Johnson, US President Joe Biden and Indian Prime Minister Narendra Modi.
They were set to acknowledge the existential threat of climate change but stop short of radical new commitments to tame global warming.
A draft statement showed major countries are only likely to slightly toughen their pledges on climate action while failing to set tough new targets that activists say are vital to prevent environmental catastrophe.
Chinese leader Xi Jinping and Russian President Vladimir Putin stayed away because of their continued concerns over COVID, dimming hopes of major progress in climate diplomacy ahead of the forthcoming COP26 summit in Glasgow.
Johnson acknowledged the talks in Rome and Glasgow would be difficult but warned without courageous action, world civilisation could collapse as swiftly as the ancient Roman empire, ushering in a new Dark Age.
He said: "It's going to be very, very tough to get the agreement we need.”
G20 countries, which account for up to 80% of the world's carbon emissions, will step up their efforts to limit global warming at 1.5 degrees Celsius - the level scientists have said is necessary to avoid disastrous new climate patterns.
A statement also said the leaders recognised "the key relevance" of achieving net-zero carbon emissions by the middle of this century - a goal some of the world's largest polluters have still not committed to.
While the climate debate will dominate in Rome, much of the first day of the summit, held in a futuristic convention centre called "The Cloud", will be given over to discuss the COVID-19 health crisis and economic recovery.
Fears over rising energy prices and stretched supply chains will be addressed.
Leaders will also endorse plans to vaccinate 70% of the world's population against COVID-19 by mid-2022 and create a task force to fight future pandemics.
Watch: G20 leaders seek consensus on climate, economy and vaccines