Paris's most famous landmark will be bathed in black, yellow and red light in homage to "the victims, their loved ones and all the people of Belgium," Hidalgo said in a statement following the attacks.
"Today Europe is targeted at her heart," she declared. "Once more it is basic values that are attacked: freedom, humanism, tolerance and unshakeable commitment to democracy."
The mayor, who earlier spoke to her Brussels counterpart Yvan Mayeur, invited Parisians to assemble in front of the city hall at 7.00pm for a silent tribute.
The French parliament observed a minute's silence at 3.00pm.
French President François Hollande spoke to Belgian Prime Minister Charles Michel by phone early Tuesday afternoon, a presidential palace statement said, to express his solidarity with the victims and share information on the fight against terror.
Suspected Paris attacker Salah Abdeslam is currently being held in Belgium, following his arrest last Friday.
Officials called on media to refrain from reporting details of the inquiry into the attacks.
"The federal prosecutor notes that certain journalists are reporting information about the investigation under way," a statement said. "The federal prosecutor urges the press to immediately refrain from conveying information relative to the investigation so as not to harm the inquiry."
"We fear that people are still at large," Belgian Foreign Minister Didier Reynders told a TV interviewer.
The explosion at Maalbeck metro station killed abpout 20 people, according to the Brussels local transport authority, with estimates of the number injured as high as 70, many of them serious.
The royal palace, which is not far away, was evacuated, according to public broadcaster RTBF.
The two blasts at the airport killed 14 people and injured 92, according to the emergency services, while the public prosecutor's office said one of them was the work of a suicide bomber.
They went off near a gate for flights for America, according to an eyewitness.
Security has been stepped up at French airports, including Paris Charles De Gaulle and Orly, as well as London Gatwick, Frankfurt and Copenhagen.
In Cairo the head of Sunni Islam's leading seat of learning, Al-Azhar, said the attacks "violate the tolerant teachings of Islam," and urged the international community to confront the "epidemic" of terrorism.