World leaders have expressed their shock at the attacks in Brussels. Here are some of the reactions:
"We are at war. We have been subjected for the last few months in Europe to acts of war." - French Prime Minister Manuel Valls.
"We realise we face a tragic moment. We have to be calm and show solidarity." - Belgian Prime Minister Charles Michel.
"The King and Queen are devastated by the attacks." - statement from Belgium's royal family.
"These attacks mark another low by the terrorists in the service of hatred and violence." - EU Council President Donald Tusk.