Story highlights Scuffles break out, police don gas masks and use pepper spray on the crowd

Eleven people are injured, police say

(CNN) At least 21 people were arrested as Trump supporters and opponents clashed Saturday at a park in Berkeley, California, police said.

Eleven people were injured, with seven transported to the hospital in unknown condition, Berkeley Police spokesman Byron White told CNN.

"A large number of fights have occurred and numerous fireworks have been thrown in the crowds," Berkeley police said in a statement. "There have also been numerous reports of pepper spray being used in the crowd."

CNN affiliate KPIX reported that Trump supporters planned a "Patriot Day" rally at noon and counter-protesters showed up a few hours earlier.

Fights broke out Saturday during pro- and anti-Trump protests in Berkeley, California.

Hundreds of people had gathered in Civic Center Park. Police set up a barrier of orange mesh fence to separate the two sides but it quickly fell down as protesters started fighting, KPIX said.