As many as 20 sleeper cells of between 120 and 180 people could be ready to strike in France, Germany, Belgium and the Netherlands, a Western intelligence source told CNN.
European Union and Middle East intelligence agencies had identified an "imminent threat" to Belgium, and possibly to the Netherlands, the source told CNN.
"There is a tremendous amount of concern over sleeper cells in Europe," said Western official with direct knowledge of the situation.
Authorities worry about a rash of copycat attacks throughout Europe, according to the official, who compared the problem to "a slow-motion car accident happening right in front of us."
Intelligence officials in Europe were trying to determine the nature of the wider threat, a senior European counterterrorism official told CNN. They were monitoring groups of men who have returned from fighting in Syria.
It is believed ISIS directed the men to return to Europe to launch attacks in retaliation for airstrikes against the terror group in Syria and Iraq, according to the official.
Overall, authorities made 17 arrests related to the Belgium threat -- 13 in Belgium and four at two locations in France.
Here are the latest developments related to the terror threat in the West:
• The suspected Islamist terrorists who had a shootout with police on Thursday in Verviers, Belgium, have ties to ISIS-linked cells in other European countries, a senior Belgian counterterrorism source told CNN's Paul Cruickshank. The two suspects who died in the shootout are believed to have fought with ISIS in Syria, the source said.
• A total of 122,000 police officers, gendarmes and military personnel are deployed across France as part of the security plan, Interior Minister Bernard Cazeneuve said.
• There does not appear to be a command-and-control element to last week's Paris attacks at the offices of Charlie Hebdo magazine and two additional locations other than a general "go forward and do something," the Western official with direct knowledge told CNN. The attacks were "highly franchised terrorism with general instructions."
• Sectarian turmoil in Yemen is bolstering al Qaeda in the Arabian Peninsula's (AQAP) local recruitment and buying it space for attacks against the West and train the next generation of bomb makers, according to one Western diplomat.
• Metropolitan Police Assistant Commissioner Mark Rowley, Britain's senior counterterrorism officer, said additional police patrols were being put in place at Jewish community centers, synagogues and schools.
• An 18-year-old woman was arrested at Stansted Airport near London on suspicion of terrorism offenses, London's Metropolitan Police said on Twitter.
• British Prime Minister David Cameron called on his country and the United States to fight the "poisonous ideology" behind the terrorist attacks that killed at least 17 people in France this month. "Britain and America both face threats to our national security from people who hate what our countries stand for and are determined to do us harm," Cameron said at a White House news conference with President Barack Obama.
• Obama promised to "do everything in our power" to assist France in its effort to combat terrorism.
• Two people suspected of involvement with the alleged terror cell in Verviers were detained trying to cross from France into Italy through the Frejus tunnel, a spokesman for Belgium's federal prosecutor's office said.
• Belgian federal prosecutor Eric Van der Sypt said 13 people were arrested in a dozen anti-terror raids
. Authorities believed the "threat of a terrorist attack was very imminent."
• The suspected terror cell, which included people returning from Syria, planned to target police officers, Van der Sypt said. Police said they recovered weapons, bomb-making materials and police uniforms.
• Belgium put 150 troops on standby for anti-terror operations.
• At least a dozen people were detained in the Paris region overnight in connection with last week's shootings in Paris, the city prosecutor's office said.
• Two men in their early 40s were arrested in Berlin on suspicion of links to ISIS
, police said. They did not appear to have been planning attacks. Berlin police spokesman Stefan Redlich said the investigation had started a year ago; the police operation planned several weeks ago.
• The Netherlands said it was not raising its terror threat level, currently at "substantial," the second-highest . "That means there is a realistic threat, but no concrete or specific information of an attack," said government spokesman Edmond Messchaert.
• The current level of security in Belgium will remain for at least one month and will then be evaluated to determine if it needs to be modified, the country's prime minister said late Friday. "From the time we are confronted with an increase in the threat, we went to level 3 on a scale of 4; we must mobilize the resources available," Charles Michel told CNN affiliate RTL.
Following up on Paris attacks
European counterterrorism agencies have been trying to identify and thwart potential threats after the deadly attacks in Paris
last week, in which 17 people were killed.
Security services are working to track those associated with the attackers: brothers Said and Cherif Kouachi and Amedy Coulibaly.
Neetin Karasular, a suspected Belgian trafficker in weapons who met with Coulibaly's widow, Hayat Boumeddiene, is in custody, Karasular's attorney, Michel Bouchat, told CNN.
Karasular was charged with association with wrongdoers and firearm offenses, in Charleroi, Belgium. But his attorney said the arrest was not connected to the other Belgian raids.
Coulibaly, who attacked a kosher supermarket in Paris last Friday, pledged allegiance to ISIS.
However, the Kouachis
, the men who French authorities say carried out the deadly shooting at the offices of the French satirical magazine, are believed to have had links to AQAP
, which has claimed responsibility
for the shootings.
A European counterterrorism official told CNN that there were indications that ISIS leadership had directed returnees from Iraq and Syria to launch attacks in Europe in revenge for Syria and Iraq airstrikes.
The official, who cited France, the United Kingdom and Belgium as countries facing a particular threat, said counterterrorism agencies in Germany are on high alert.
Several European nations, including the United Kingdom, France, Belgium and the Netherlands, are participating in the air campaign against ISIS in Iraq.