America celebrating Fourth of July on guard

Last Updated Jul 4, 2015 7:36 AM EDT

Security officials were on alert Saturday though there's been no specific threat aimed at this Fourh of July weekend from the Islamic State of Iraq and Syria, or ISIS, or other terror groups.

But there is still concern about the possibility of attacks by so-called "lone-wolf" terrorists.

Security measures to prevent lone wolf attack across U.S.

The FBI and Department of Homeland Security have issued a bulletin warning of potential attacks through the July Fourth weekend, leading to a visible - and invisible - response at key events, CBS News' Mark Albert reports.

America will celebrate its 239th birthday on guard.

ISIS has called for attacks against members of the military, law enforcement, the U.S. government and the American public.

"Yes, there's been a lot of chatter," said Everett Gillison, Philadelphia deputy mayor for public safety. "That's something that you've heard all over the country, but there's no specific threat to anywhere that we're aware of right now in the country. It just is that we have to be vigilant."

NYPD's John Miller on threat of terror during July Fourth weekend

In Philadelphia, the birthplace of the Declaration of Independence, Gillison said the city is relying on a joint operations center to oversee coordination among its officers and departments.

"We never let anyone dictate to us how we're gonna live our lives," he said.

In New York City, 7,000 extra officers will be on the streets with radiation detectors on the Hudson River and in the air.

In the nation's capital, 700,000 people are expected on the National Mall; all will be screened.

FBI and Homeland Security warn of potential July 4th terror attacks

And Chicago police are working 12-hour shifts.

The FBI has set up coordination centers around the country with special TSA teams assigned to large air travel hubs.

"I just want to wish everyone a happy and healthy Fourth," said Boston Mayor Marty Walsh, "and ask the people of Boston to remain safe, vigilant and keep their eyes open."

Walsh and Boston Police Commissioner William Evans said Friday the city is on the lookout for attackers who might act on their own.

"Obviously everyone's concerns have been raised because of all the ISIS activity that's going around and because of, you know, the marathon bombing," said Evans. "We're just stepping it up just to make sure it can be as secure as possible."

The commissioner added that a lone-wolf attack is his worst nightmare.

Meanwhile, recent attacks in France, Kuwait and Tunisia also highlight the ongoing risk to American embassies and interests abroad.