Police in Belguim have detained 21 people in 29 raids since yesterday, but Paris fugitive Salah Abdeslam remains on the run, prosecutors have said.

Federal prosecutor Eric Van Der Sypt said "no firearms or explosives were discovered" in the raids - 19 in Molenbeek and other Brussels boroughs and three in Charleroi in the country's south yesterday and five raids in the Brussels area and two around the eastern city of Liege today.

One of those detained was injured when a car he was in tried to ram police during an attempted getaway.

Mr Van Der Sypt said "certain elements" in the investigation made yesterday's intervention necessary. "The investigation will in any case be relentlessly continued," he said.

Yesterday's raids capped a tense day with hundreds of troops patrolling and authorities hunting for one or more suspected militants, the Belgian government chose Sunday to keep the capital on the highest state of alert into the start of the work week to prevent a Paris-style attack.

Citing a "serious and imminent" threat, prime minister Charles Michel announced that schools and universities in Brussels would be closed today, with the tube network remaining shut down, preventing a return to normal in the city that is also home to the European Union's main institutions.

"We fear an attack like in Paris, with several individuals, perhaps in several places," Mr Michel said after chairing a meeting of Belgium's National Security Council.

While Brussels was kept on the highest of four alert levels, the rest of the country remains on a Level 3 alert, meaning an attack is "possible and likely".

"Nobody is pleased with such a situation. Neither are we. But we have to take our responsibility," Mr Michel said.

Western leaders stepped up the rhetoric against the Islamic State (IS) group, which has claimed responsibility for the attacks in Paris that killed 130 people and wounded hundreds more; the suicide bombings in Beirut that killed 43 people and injured more than 200; and the downing of the Russian jetliner carrying 224 people in Egypt's Sinai Peninsula. All happened within the past month.

"We will not accept the idea that terrorist assaults on restaurants and theatres and hotels are the new normal, or that we are powerless to stop them," US president Barack Obama said in Kuala Lumpur, Malaysia.

French defence minister Jean-Yves Le Drian said IS must be destroyed at all costs. "We must annihilate Islamic State worldwide ... and we must destroy Islamic State on its own territory," he said. "That's the only possible direction."

The decision to put Brussels on the highest alert came early on Saturday as authorities frantically searched for Abdeslam, who is believed to have played a key role in the November 13 attacks in France. He is known to have crossed into Belgium the day after the attacks.

Interior minister Jan Jambon warned that the threat would not necessarily disappear if Abdeslam was found, because they were looking for several people in connection with a possible planned attack in Brussels.

"The terror threat is wider than just that person," he said. "We are looking at several things. That is why we are making the big show of power and following everything up by the minute. It's of no use to hide this."

Several of the Paris attackers had lived in Brussels, including Abdelhamid Abaaoud, the plot's mastermind, who was killed on Wednesday in a shoot-out with French police.

Abdeslam is known to have crossed into Belgium on November 14. His brother, Mohamed Abdeslam, went on Belgian TV and urged him to surrender, saying he would rather see him "in prison than in a cemetery".

Authorities in Turkey said on Saturday that a 26-year-old Belgian citizen suspected of being linked to Islamic extremists and possibly to the Paris attacks had been detained in the coastal city of Antalya.

France has intensified its aerial bombing in Syria and the aircraft carrier Charles de Gaulle, which has been sent to the Mediterranean to help combat IS militants in Syria, will be "operational" from today and "ready to act".

French president Francois Hollande will meet British prime minister David Cameron in Paris today and will travel to Washington and Moscow later in the week to push for a stronger international coalition against IS.