The bulletin, which marks the addition of a new level of public warning to the system, will be in effect for the next six months, or until events dictate otherwise, Johnson said.
The Department of Homeland Security is "especially concerned that terrorist-inspired individuals and homegrown violent extremists may be encouraged or inspired to target public events or places," the bulletin stated.
"As we saw in the recent attacks in San Bernardino and Paris, terrorists will consider a diverse and wide selection of targets for attacks,'' the DHS notice said.
There was no specific information, however, about a pending attack, Johnson said.
Under the amended advisory system, periodic bulletins will be issued to inform the American public of more general threats identified by national security officials.
The change is designed to provide a more flexible way for federal authorities to communicate the existence of potential threats short of the two-tiered system the department had been using, which would only activate warnings in the event of credible or imminent terrorist threats against the United States.
The National Terrorism Advisory System (NTAS), which replaced the much-maligned color-coded matrix created in the aftermath of 9/11, had never been activated since its creation in 2011 because of its high-threshold requirements.
Johnson said Wednesday that national security officials considered activating the system early this year in response to an undisclosed threat stream. But he said the threat information was not specific enough to meet the system requirement.
"People are anxious now,'' the secretary said in the announcement just two weeks after the San Bernardino shootings that left 14 dead and 22 wounded. The married killers, Sayed Farook and Tashfeen Malik, died in a shootout with police hours after their rampage. "People need to know what their government is doing to protect the homeland.''
Federal investigators believe Farook and Malik were radicalized years before launching the attacks earlier this month. It remains unclear, however, why they chose to target a holiday party packed with many of Farook's San Bernardino County co-workers. Authorities have been closely questioning Farook's former neighbor in Riverside, Calif., who provided the couple with two rifles used in the attack.
In the bulletin issued Wednesday, the public was urged to report suspicious activity to authorities and offered guidance for how community leaders, co-workers and family members may recognize "signs of potential radicalization to violence.''
With much of the country in the midst of holiday celebrations, the notice also said that "more stringent security should also be anticipated at public places and events.''
"This may include a heavy police presence, additional restrictions and searches on bags and the use of screening technologies,'' the bulletin stated.
Johnson signaled the system change as recently as last week, saying that the recent attacks illustrated "a new phase in the global terrorist threat'' that includes both terrorist-directed assaults and those inspired by organizations that involve singer attackers or small groups who can often evade law enforcement detection.
Authorities maintained that the system change was not prompted by any one recent event. Johnson had directed a review of the existing system earlier this year. The changes are the result of that review.