by Peter Mertz

DENVER, the United States, Dec. 25 (Xinhua) -- Heightened security from coast to coast paved the way for a relatively calm and peaceful Christmas in the United States, as families gathered to pray, exchange gifts and celebrate family traditions.

Law enforcement authorities had stopped at least two mass killing attempts before the holiday season started.

On the West Coast last week, the FBI foiled a plot targeting downtown San Francisco's popular tourist destination Pier 39, that could have killed hundreds of Christmas revelers.

The FBI arrested Everett Jameson, a tow truck driver-turned Islamic State follower alleged to have been plotting the attack, in nearby Modesto, California, on Dec. 19.

The 26-year-old is said to have had written a letter saying: "You've allowed Donald J Trump to give away Al Quds to the Jews," a reference to the U.S. president's controversial decision to recognize Jerusalem as the capital of Israel.

The authorities were alerted by Jameson's online support for the Oct. 31 truck attack in New York City in which eight people were killed and eight more injured by an Uzbek emigrant.

Jameson had just started working as a tow truck driver and the FBI said he may have been planning a similar attack.

In the East Coast state of Pennsylvania, police prevented another mass murder. Ahmed Aminamin El-Mofty, 51, was shot and killed by police on Friday after he fired at law enforcement officers in various parts of the city.

On Sunday, the Department of Homeland Security officially called the shootings targeting police in Pennsylvanian capital Harrisburg a terrorist attack.

Los Angeles Police Department deputy chief Michael Downing said his counter-terrorism bureau was especially aware of the potential danger from attackers using trucks or other large vehicles to mow down pedestrians.

People across the country lamented having to spend the major holiday season under tight security.

"Sadly, on this day that we honor the Prince of Peace, we are less safe than ever before in our country's history," Maryland history teacher Douglas Hubscher said.

Hubscher told Xinhua that the availability of military weapons and various U.S. policies incurring the wrath of extremists across the world had made America vulnerable, especially on Christmas.

In New York City, sniffer dogs and heavily armed police officers were deployed in St. Patrick's Cathedral in Midtown Manhattan, where thousands of worshippers flocked to attend Christmas Mass on Monday.

"I was going to see the ball drop on New Year's Eve, but just to be in that crowd after what has happened in Vegas and then, what happened here in New York recently, I said I'm not going to go in that crowd," Rhonda Mack of Murrieta, California, told ABC News.

There are also voices bemoaning the increasing commercialization of the Christmas holiday.

"When we were kids, America started Black Friday and the mega-commercializing of Christmas, much to the dismay of my Catholic community," Hubscher, 58, told Xinhua.

"That was the beginning of the downfall of Christmas, and with the Trump administration, we are seeing another nail in the Christmas coffin," he said.