On Tuesday afternoon, people gathered on the steps of Brussels' old stock exchange building with a banner that read "I am Brussels" in both French and Flemish. Vigils and gestures of solidarity popped up in cities around the world as people quietly left flowers and lit candles amid heightened security. 

BELGIUM-BLAST global reaction I am Brussels March 22 2016

(Charles Platiau/Reuters)

Fellow EU countries lit up their monuments in solidarity. 

The Eiffel Tower in Paris and the Trevi Fountain in Rome were lit up in black, yellow and red, the colours of Belgium's flag, in tribute to the victims of Tuesday's bombings. 

BELGIUM-BLAST global reaction Tour EIFFEL March 22 2016

(Philippe Wojazer/Reuters)

BELGIUM-BLAST global reaxtion Rome ITALY March 22 2016

(Stefano Rellandini/Reuters)

In Prague, candles were left outside the Belgian Embassy.

BELGIUM-BLAST global reaction CZECH Republic vigil March 22 2016

(David W. Cerny/Reuters)

In Berlin and Moscow, the flowers piled up.

Belgium attacks global reaction Germany March 22 2016

(Sean Gallup/Getty)

BELGIUM-BLAST global reaction RUSSIA memorial March 22 2016

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World leaders expressed condolences and condemned the attackers.

Prime Minister Justin Trudeau, hours before unveiling the first budget of his Liberal government, called the "terrorist attacks in Brussels" a "deplorable" act. 

BELGIUM-BLAST global reaction PM Justin TRUDEAU March 22 2016

(Chris Wattie/Reuters) (Chris Wattie/Reuters)

Barack Obama gave a similar message from Cuba.

U.S. President Barack Obama, pictured with National Security Adviser Susan Rice, took a call with the Department of Homeland Security in the last hours of his historic visit to Havana. During a speech from the Cuban capital, Obama said "the thoughts and prayers of the American people are with the people of Belgium."

BELGIUM-BLAST global reaction Barack Obama in Cuba March 22 2016

(Pete Souza/White House/Reuters)

Cities across Europe tightened security in the wake of the attacks.

In Britain and in France, as in Brussels, thousands of additional police officers were deployed at international transport hubs and popular gathering spots.

BELGIUM-BLAST global reaction security March 22 2016

(Christian Hartmann/Reuters)

Belgium raised its terror alert to the highest level.

Planes and trains were diverted away from the city and orders to stay put for most of the workday were issued in the immediate wake of the attacks. 

Brussels attacks global reaction London UK security March 22 2016

(Adrian Dennis/AFP/Getty)

It was a similar scene in Washington, D.C. 

Heavily armed police with canine units patrolled Union Station in Washington on Tuesday after the bombings in Brussels. 

Belgium attacks global reaction Washington DC security March 22 2016

(Win McNamee/Getty)

Meanwhile, a Belgium-wide manhunt is underway. 

ISIS has claimed responsibility for the pair of early morning blasts at the Zaventem airport, as well as a bombing at one of the city's metro stations, that left at least 30 people dead and as many as 190 others wounded.

BELGIUM-BLAST manhunt underway March 22 2016

(Christian Hartmann/Reuters)

With files from Reuters