Philadelphia Police Commissioner Charles H. Ramsey Saturday night expressed anger over the execution-style shooting deaths of two New York City policemen, and said his officers needed to be careful and stay alert.

Ramsey, who leads President Obama's Task Force on 21st Century Policing, urged those leading protests of the deaths of Eric Garner in Staten Island and Michael Brown in Ferguson, Mo., to "at least acknowledge the tragedy that occurred tonight and call for calm and not let this escalate any further."

The commissioner said he worries about "the environment that we are in now, with . . . some of the people chanting 'We want dead cops.' You just don't know who that impacts, and now you have two officers dead, leaving behind families."

John McNesby, president of the local Fraternal Order of Police lodge, agreed that officers "have to be a little extra careful now," adding that the protests give edge to an already dangerous job.

"It's a fragile time," he said. "Cops have to be careful and watch each other's backs."

He said Philadelphia's officers would assist the New York force in any way possible: "It's a brotherhood, whether it's Philadelphia, or New York, or whatever. We all stick together."

-Mike Newall and Jonathan Lai