Pruitt's early departure from the discussions may further obstruct plans by environment ministers at the two-day meeting.
Pruitt said in a statement later on Sunday that the United States had always been a world leader on environmental stewardship and "that was demonstrated on a global stage today".
United Nations executive director for the environment Erik Solheim, present at the morning sessions, said G7 countries, excluding the United States were absolutely committed to move ahead with climate action whatever happened in the White House.
"So far there's only been an announcement that the United States is withdrawing, it has not yet materialised". The Environmental Summit being held on Sunday and Monday is scheduled to discuss issues that include climate change, sustainable development, and litter at sea, all important issues to everyone and encompassing more than just the environment, but health and the global economy.More news: Woman 'chained like a dog' by serial killer for months
Differences between the United States and other leading economies over climate change remain wide and are destined to stay that way, Italy's environment minister Gian Luca Galletti said on Sunday. "A lot of people are very upset about Trump's decision and it has started a new debate", Giacomo Cossu, one of the organisers of the demonstration, told AFP.
"Trump has revealed the truth that lies behind the rhetoric of the G7 on the environment". That is not our model.
Chaperoned by hundreds of riot police, the demonstration appeared to be passing off peacefully with protestors brandishing placards declaring: "There is no Planet B" and "They think the Kyoto protocol is a Japanese erotic film".
Trump has said that his administration would begin negotiations either to re-enter the Paris deal or set up a new agreement on "terms that are fair to the United States", which is the world's second biggest carbon emitter behind China. Mr Trump has said Washington will not be bound by the targets on reducing emissions of greenhouse gases set down in Paris, and will cut funding for developing countries.
Important players in U.S. industry and individual cities and states are already implementing changes aimed at meeting the targets laid down in Paris, where most of the world's countries agreed to try to cap global temperature rises at two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels. The G7 Bologna talks are to wrap up on Monday.