"We are very alert and call for caution," he said. "The potential targets remain the same — shopping centers and shopping streets and public transport. We want to return to a normal way of life as quickly as possible."
The unprecedented security measures come as authorities hunt for Salah Abdeslam, a fugitive since being named as a suspect in the Paris attacks. He has yet to be found.
Belgian authorities arrested a suspect on terrorism-related charges Monday after a series of raids that saw 21 people detained from Sunday night through midday Monday.
The federal prosecutor said the suspect, who was not identified, was charged with terrorism offenses related to the Paris attacks after authorities detained 16 people Sunday. Three others were charged with similar offenses last week. The other 15 people detained Sunday evening were released. Federal prosecutor Eric Van Der Sypt said most of the raids were carried out Sunday in Molenbeek, a Brussels suburb home to many of the Paris attackers.
Earlier in the day, British Prime Minister David Cameron said in Paris that he will ask for parliamentary approval for the U.K. to join airstrikes against ISIL in Syria. If the strikes are approved, the U.K. will join other nations carrying out bombings in Syria, including the U.S., France and Russia.
“I firmly support the action that [French] President [François] Hollande has taken to strike ISIL in Syria, and it is my firm conviction that Britain should do so too,” Cameron said, adding that that was a decision for Parliament to make.
“We are convinced that we must continue to strike ISIL in Syria. We will intensify our strikes. We will choose the sites that will cause the most damage to this terrorist army,” Hollande said Monday.
Cameron and Hollande paid a quiet visit to the Bataclan concert venue, which saw the worst of the carnage in Paris. Hollande plans to meet Tuesday with President Barack Obama, then Wednesday with German Chancellor Angela Merkel and with Russian President Vladimir Putin on Thursday in pursuit of a unified strategy in Syria.
France's Defense Ministry said it launched its first airstrikes from the Charles de Gaulle aircraft carrier, bombing ISIL targets in the Iraqi cities of Ramadi and Mosul in a seven-hour operation. The ministry said that four Rafale fighter jets were sent from the carrier on Monday afternoon, with two flying over each city. France has already carried out strikes against ISIL targets in Syria.
Britain has been carrying out airstrikes in Iraq, and Cameron has long wished for an expanded mandate to include targets in Syria. But having lost a vote once two years ago, his government had been reluctant to even suggest a vote until it could be certain it would win.
The raids in Belgium began late Sunday, capping a tense weekend that saw hundreds of troops patrolling streets.
Several of the Paris attackers lived in Brussels, including Abdelhamid Abaaoud, the man who authorities say orchestrated the plot. He was killed Wednesday in a standoff with French police.
French police issued a new public appeal for help in identifying the third attacker who was killed in the assault at the national stadium. They posted a photo of the man on Twitter on Sunday, asking the public for information.
Greek police said that the man posed as an asylum seeker before the carnage. Public Order Minister Nikos Toskas said the man traveled to the island of Leros on Oct. 3, but he gave no further details.
Two senior Greek law enforcement officials told The Associated Press that the man traveled with another attacker identified as Ahmad al-Mohammad. Both men were rescued by the Greek coast guard while traveling from nearby Turkey on a boat carrying nearly 200 migrants and refugees that sank before reaching Greece.
France has extended a state of emergency that allows police raids, searches and house arrest without permission from a judge, for three months. On Saturday it extended a ban on demonstrations and other gatherings through Nov. 30, when a U.N. climate conference with more than 100 heads of state is scheduled to start.
ISIL has claimed responsibility for the Paris attacks, the suicide bombings in Beirut that killed 43 people and injured more than 200 and the downing over Egypt's Sinai Peninsula of a Russian jet carrying 224 people. All happened within the past month.
Al Jazeera and The Associated Press