All public transport in Brussels was initially shut down, as it was in London during 2005 Islamist militant attacks there that killed 52. Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".
Brussels airport will remain closed on Wednesday, its chief executive Arnaud Feist told reporters.
'We are at war'
“We are at war and we have been subjected to acts of war in Europe for the last few months," French Prime Minister Manuel Valls said.
Train services on the cross-channel tunnel from London to Brussels were suspended. Britain advised its citizens to avoid all but essential travel to Brussels.
Security services have been on a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.
While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.
Belgian flag to light up Eiffel Tower
The Eiffel Tower is to be lit up in Belgian colours as a symbol of support in Paris for the people of Brussels after twin attacks there killed at least 34 people.
"In solidarity with the people of Brussels, Paris will light up the Eiffel Tower this evening in the colours of the Belgian flag," Paris Mayor Anne Hidalgo said on her Twitter account on Tuesday.
The attacks took the form of explosions at Brussels airport and a rush-hour metro train in the Belgian capital earlier on Tuesday, triggering security alerts across western Europe and bringing some cross-border transport to a halt.
They were a strong reminder to Parisians of the November 13 Islamist militant attacks in Paris where 130 people died, and came four days after the arrest in Brussels of a suspected participant in those assaults.
French cartoonists have also shown their solidarity with Belgium by picking up their pens in a move which echoes the response to the Charlie Hebdo terror attacks in Paris.
French newspaper Le Monde published an image of a weeping French flag with a human face putting his arm around his Belgian neighbour.
And circulating on Twitter has been a picture of beloved Belgian fictional hero Tintin crying as he reads a newspaper reporting on the explosions in Brussels.
Another image shows the character shedding tears coloured in the black, yellow and red of the Belgian national flag.
