Police cordoned off 16 square blocks in downtown Portland to all vehicle and pedestrian traffic Monday evening as protesters began their fourth consecutive night of demonstrations following the killing of George Floyd.

"We are here to support the expression of their First Amendment rights," the bureau wrote in a tweet Monday night. "The area behind the fencing is currently closed and to not interfere with fencing. Thank you for your cooperation!"

The barrier encompasses what has been a hot spot for protesters for the past three nights: westward from Southwest 1st Avenue to 5th Avenue and from Southwest Jefferson Street up to Taylor Street. Locking down these blocks appears to be an effort to keep protesters from assembling in front of the Multnomah County Justice Center—the eventual scene of showdowns with police on all three previous nights of marching.

The barricade consists of chain-link fence, with garbage trucks and water trucks parked along the fence's interior.

"The closure of this area is due to ongoing public safety concerns and ongoing criminal acts," the Police Bureau wrote on its website. "Violations of this order may result in arrest."

Today, Gov. Kate Brown agreed to deploy 50 members of the Oregon National Guard to assist police, but said the troops would not be armed or make arrests.

Protesters have so far assembled on the east side of the Willamette River, in the parks surrounding Revolution Hall.