The mostly peaceful demonstrations in Quebec City on Thursday saw thousands of protesters took to the streets, with a heavy police presence deployed in the area ahead of the summit.
Twitter users posted pics appearing to show some of the police carrying assault rifles, with many in riot gear.
Protesters included left-wing and anti-globalization activists, as well as native Canadian ("First Nations") activists protesting their tribal leaders' participation in the G7 welcoming ceremony, set for Friday.
Police made at least two arrests. A CBC reporter said the suspects are accused of possessing explosive substances.
All in all, Thursday's protests were mostly peaceful, but local stores have barricaded windows and closed early, just in case Friday's protests become violent.
This year's G7 meeting is fraught with political turmoil amid trade disagreements between the US and its Canadian and European partners. On Thursday, French President Emmanuel Macron tweeted about President Trump's "isolation" and said the other members of the Group of Seven would be ready to sign a six-country agreement "if need be."
Trump fired back, going after Macron and Canadian Prime Minister Justin Trudeau and accusing Brussels and Ottawa of "totally unfair" tariffs and trade barriers, which hurt American farmers, workers and companies.
The summit will officially kick off on Friday in the provincial Quebec town of La Malbaie and run until Saturday. President Trump is expected to leave the summit early to fly to Singapore, skipping the climate session, and reportedly declining a one-on-one meeting with British Prime Minister Theresa May.