Burlington Police said "defensive munitions" - including 8-10 pepper balls and a sting ball round - were fired to protect two officers from protesters blocking busses that were attempting to pick up conference participants for dinner.
They said the officers suffered minor injuries. During the skirmish, at least two protesters were hit by the pepper balls or the sting ball. Another person was sprayed with pepper spray, police said. There were no arrests or other reports of injuries.
Avery Pittman of Burlington, spokeswoman for participating protest groups, said protesters were standing in front of slow-moving busses thought to be carrying conference participants when police used rubber and pepper bullets.
Police denied using rubber bullets but described the sting ball round as having several small rubber "stingball pellets."
Raw video posted the website of the Burlington Free Press appears to show an officer shooting a protester twice at close range.
In a three-page statement, police said a small group of protesters "chose to actively inhibit" the movement of 200 people headed to a dinner event.
Numerous warnings were issued, police said, and by about 5 p.m. some protesters began blocking the busses by sitting or lying on the ground.
Police said a protester dragged an officer
Protesters were fired upon, police said, "to protect officers from those in the crowd who were moving toward them."
Earlier Sunday, an estimated 500 to 600 people attended a rally at City Hall before marching through downtown to protest government policies toward the environment, immigrants and refugees, and the economy, protest organizers said.
Protesters were also creating a "human oil spill" at Battery Park to decry the possibility that tar sands oil from western Canada might be shipped across Vermont, New Hampshire and Maine. Members of the Occupy movement were also holding a regional gathering at the park in the afternoon and evening.
Regional leaders attending the governors and premiers conference had been expected to begin arriving in Burlington on Sunday for meetings Monday.
The protest events, dubbed "Convergence on the Conference," also included a community dinner and presentations from different groups on Sunday, with more events scheduled Monday.
While protesters speak out on a variety of issues ranging from power transmission lines and oil pipelines to Vermont Yankee nuclear power plant, the governors and premiers and their staffs will be meeting "behind closed doors" to discuss issues that have wide-ranging impacts, Pittman said.
"It's clear the governors and premiers are meeting to talk about trade policy, energy and infrastructure," Pittman said. "They're definitely prioritizing profits and money-making over the needs of the people or the impact these proposals will have on us, the people who live on the land and are affected by the decisions."