Thirty-four people were arrested at the parade, said Boston Police Officer James Moccia, a department spokesman. Four officers also suffered non-life threatening injuries, he said.
The parade started around noon at Copley Square and moved down Boylston and Tremont streets, ending at City Hall Plaza.
Floats and signs expressed pro-military and pro-Trump sentiments, such as “Support our troops” and “Build the wall and crime will fall.” A man in a jester’s hat carried a sign saying “Great to be straight” as he danced down the street.
“I’m outraged by the idea that straight people need a pride parade,” said parade watcher Shoshanna Ehrlich, who came with her daughter. “We are not an oppressed majority by an stretch of the imagination. This is full of hate and offensive.”
Monica Cannon-Grant Ally, with Black Lives Matter Cambridge, helped organize one of the counterprotests, said protesters wanted to “stand up against hatred.”
“I feel like they capitalized on Boston Pride, which is our LGBTQ celebration. They used it as their next thing to hate on,” she told CNN in an interview.
“We chanted against hatred and we won,” said Ally, the founder and CEO of Violence in Boston.
Nine people were transported to area hospitals, Boston EMS spokeswoman Caitlin McLaughlin said. She could not provide information on their conditions.
“The Straight Pride Event will be held to achieve inclusivity and spread awareness of issues impacting straights in Greater Boston and beyond,” the group’s website said.
Milo Yiannopoulos, a right-wing political commentator and writer, was the grand marshal.
In a tweet, Boston Mayor Marty Walsh he would be celebrating “what makes our city so great — our people — by attending neighborhood block parties, visiting farmer’s markets and BBQs, and celebrating safe neighborhoods.”
“As I’ve said, a ‘Straight Pride’ Parade won’t overshadow the tremendous role Boston plays in the national movement for equality,” Walsh said via Twitter.
CNN’s Monica Haider and Darran Simon contributed to this report.