Some 20,000 people gathered for a march that was initially peaceful, but protesters later targeted what they depicted as symbols of capitalism and multilateralism, including the parked Tesla and the UN office.
Demonstrators ripped bricks from the ground to throw at police, while children cried as teargas wafted over downtown Geneva's sun-baked streets, Reuters witnesses said.
Protests have been common at G7 gatherings over the years, with many demonstrators using the summits to decry capitalism, globalisation, climate change and inequality.
Demonstrators said they came to protest against the G7 as a symbol of concentrated political and economic power.
Last week Tesla owner Elon Musk, who has worked as an adviser to US President Donald Trump, became the world's first trillionaire.
"To me, it's a meeting of the rich that shows once again how the rich can become even richer while the poor are left behind," said protester Pippa Saugy.
The June 15–17 G7 summit in Evian-les-Bains, on the shore of Lake Geneva, will bring together the leaders of France, Britain, Canada, Germany, Italy, Japan and the United States, alongside the European Union.
Wars in the Middle East and Ukraine are set to dominate the agenda, while leaders will seek to avoid a clash with Trump as he seeks to finalise a framework peace deal with Iran.
In Geneva, businesses were boarded up and hundreds of riot police were deployed in the streets amid concerns about violence.
Mattia Piccard bristled at the strong police presence.
"This is an attempt to frighten demonstrators, to frighten people and discourage them from coming out to protest," Piccard said.
Clélia Colin, another demonstrator, said she wanted to raise the issue of gender inequality.
"The values represented by the G7 are completely misogynistic, and they contribute to inequality," said Colin.
