By Eric M. Johnson

SEATTLE (Reuters) - Two activists climbed atop an excavator on Monday, one locking himself to the equipment, in a protest over the building of an animal-testing facility at the University of Washington, halting construction at the site, sympathizers and police said.

One activist is "currently locked to the hydraulic lift at the top of the piece of heavy equipment, from which he has hung a banner reading: "YOU WILL NOT BUILD THIS LAB,'" according to a website tracking opposition to the proposed Animal Research and Care Facility.

"These animals feel pain, they feel fear, and their lives are spent confined in cages or as tools in cruel experiments," said Oliver McCaughran, the activist locked to the excavator, according to the website.

A University of Washington spokesman did not immediately respond to a request for comment, nor did construction company Skanska.

"They have been advised they're under arrest for criminal sabotage," University of Washington police chief John Vinson said of the activists. "When they come down, they'll be arrested."

"We are concerned about their safety and are hoping for a peaceful resolution," Vinson added. He said it appeared at least one activist was locked to the equipment.

The protest follows a march last month by about 500 demonstrators angered over a university process they say is not sufficiently open to public input and scrutiny, said Ben Jones, an activist connected with the opposition campaign.

McCaughran had his hands inside a lockbox, with the second activist there to provide support, such as providing him with drinking water, Jones said.

The Seattle-based university's Board of Regents unanimously approved moving forward with the new multi-species Animal Research and Care Facility in November 2013.

The building was being readied for occupancy for August 2017, the university said. It will be able to house many different species, mostly rodents such as mice and rats, as well as smaller numbers of larger animals such as rabbits, pigs and non-human primates.

"The new building will allow the University to increase the size of its research program and allow more investigators to pursue new scientific and medical advances which benefit both human and animal populations," it said.

(Reporting by Eric M. Johnson in Seattle; Editing by Peter Cooney)