After four months on the run Paris attacks suspect Salah Abdeslam remains in custody while he is treated for gunshot wounds sustained during his arrest.
The alleged terrorist was captured in a huge anti-terror operation in Brussels on Friday.
Belgian prime minister Charles Michel confirmed the news at a press conference alongside French president Francois Hollande hours after heavily armed officers stormed a building in the city's Molenbeek district.
He said two other men were also held in the operation in a part of the Belgian capital some of the Paris attackers, including suspected ringleader Abdelhamid Abaaoud, were from.
Mr Hollande congratulated the Belgian authorities and said moves would start to extradite Abdeslam, who was born in Brussels, back to France.
Mr Hollande said: "I have a special thought for the victims of the attacks on November 13 in Paris, because Salah Abdeslam is directly connected to the preparation, organisation and ... the perpetration of these attacks.
"I also think of the families who have been looking forward to these arrests, whether from close range or long distance, who are connected to that abomination."
Belgian migration minister Theo Francken confirmed the arrest, tweeting: "We've got him."
Mr Michel left the EU-Turkey migrant crisis summit amid reports of the raid.
TV footage showed armed officers descending on the area and gunshots and explosions were reported.
Fire engines and ambulances were seen driving into the gated complex, which remains under armed police guard, and a helicopter hovered overhead.
Members of the public also gathered at the scene.
Local media said police, who were joined at the scene by the army, appeared to use grenades while witnesses suggested white smoke could be seen coming from a property.
At least 10 gunshots were reportedly heard.
The news came after Belgian authorities said a man shot dead earlier this week was probably an accomplice of Abdeslam.
Fingerprints found at the address where Algerian national Mohamed Belkaid was killed suggest Abdeslam had been there too.
Two men escaped from the apartment during the gunfight with police and one of them is now thought to have been Abdeslam, 26, who fled from Paris after the terror attacks in November which killed 130 people.
Mr Hollande said the fight against terrorism would go on, and there were "connections that lead us all the time to Syria".
He said: "Although this arrest is an important stage it is not the final conclusion of this story because there have been arrests already. And there will have to be more because we know that the network was quite widespread, in Belgium, in France, in other countries of Europe as well.
"So until we have arrested all those who took part or contributed, financed that network, that terrorist network that committed the abominable attacks, the war acts of November 13, our fight will not be over until then."
Mr Michel had earlier said that he had spoken to US president Barack Obama, tweeting a picture of him with Mr Hollande and the message: "Congratulations from @POTUS (President of the United States) Belgium and France stand united in the fight against terrorism."
The White House issued a statement, saying: "The president commended the work of Belgian security services and noted that this significant arrest was the result of hard work and close co-operation between Belgian and French law enforcement authorities.
"The president reiterated that the United States stands together with Belgium and France in our shared struggle against the scourge of terrorism and once again pledged the full co-operation and support of the United States in these efforts, reaffirming our shared commitment to degrading and destroying Isil."