Mayor Martin J. Walsh has fired a 25-year-old city youth worker after learning the Roxbury woman was among 11 protesters who chained themselves to 1,200-pound, concrete-filled barrels in Milton on Thursday, snarling morning rush-hour traffic and stalling at least two ambulances, the Herald has learned.

“As mayor, you have to make tough, difficult decisions. This is not a difficult decision,” Walsh said. “This was not based on the fact of this woman protesting. It was based on the fact of putting the public safety of other people at risk.”

Nelli Ruotsalainen, a part-time youth communication specialist for the Boston Centers for Youth and Families, was noncooperative and “pled the Fifth” at her City Hall hearing yesterday, after which a Labor Relations committee recommended she be terminated, according to city officials.

Walsh said he fully supports the right to demonstrate but these protesters crossed the line — and endangered and disrupted people’s lives. The mayor said he was particularly upset that the protesters jeopardized the life of an 83-year-old car crash victim whose ambulance had to be rerouted from Boston Medical Center to a Brockton hospital.

“You can’t put people in harm’s way like that,” he said.

A separate group of 18 protesters blocked traffic by creating a “human chain” of bodies linked together with pipes, ropes and chains on I-93 in Medford. All 29 protesters were arrested and arraigned Thursday.

A press release they sent out claimed that while none are African-American, they were supporting Black Lives Matter, an activist group that has staged rallies in response to police killings of unarmed black men in Ferguson, Mo., and Staten Island, N.Y.

Ruotsalainen, a Finland native, could not be reached for comment yesterday. She pleaded not guilty in Quincy District Court to seven charges: trespassing, throwing glass on a public way, willfully obstructing an emergency vehicle, conspiracy, disorderly conduct, resisting arrest and throwing an object on a public way.

The mayor said the contract employee called her supervisor Thursday morning requesting an unpaid personal day.

“She was not terminated because she protested, because if you take a personal day you can do what you want on that day. She was terminated because of the risk of public safety. … The way the events unfolded (Thursday) really bothered me,” Walsh said.

First Amendment lawyer Harvey Silverglate said protesters’ rights to free speech “go out the window” when they cause such massive disruption — and municipal employees have fewer such rights than regular citizens. “Even an ordinary citizen,” he said, “could not be protected from that kind of disruptive activity.”