French President Emmanuel Macron called for the summit to be held less than a year after Donald Trump withdrew the United States from the Paris climate deal in June.
Mr Trump was not among the invitation-only guests in attendance.
Arnold Schwarzenegger said he would continue to fight climate change, despite United States Donald Trump pulling the US out of the Paris agreement.
'We must eliminate pollution'
The Hollywood movie star and former California governor issued a call to action, urging environmentalists to focus on reducing pollution.
"We must talk about the health aspects," he said.
"This is what people can relate to. People want to survive. That is the way the human brain is wired. If you say that eventually our glaciers are melting, the iceberg is melting, the North Pole is going to melt, the sea levels are rising in 20 years ... people can not relate to that."
"When you talk about how many people die every year, when you tell them nine million people die every year - 19,000 every day ... then they get the message."
'We will fight'
Mr Schwarzenegger met with Paris mayor Anne Hidalgo as part of his role as the founder of R20, a Geneva-based non-profit group promoting action by regional, state and local governments.
He said advocates should not be disheartened by Mr Trump's withdrawal from the Paris accord.
"Donald Trump pulled Donald Trump out of the Paris agreement, so don't worry about that," he said.
"We at a sub-national level are going to pick up the slack and continue on. We will fight and we will create the kind of future for our children and grandchildren because that is our responsibility and no one will stop us."
Mr Schwarzenegger said people should take inspiration from the coalition of 12 cities to ban the use of gasoline and diesel vehicles from large parts of their cities by 2030.
The cities include London, Los Angeles, Paris, Mexico City, Seattle, Copenhagen, Barcelona, Vancouver, Milan, Quito, Cape Town and Auckland.
"It doesn't matter that Donald Trump backed out of the Paris agreement because the private sector didn't drop out, the public sector didn't drop out, the universities didn't drop out, the scientists didn't drop out, the engineers didn't drop out ... No one else dropped out," he said.
