In an address to the nation King Philippe said March 22nd is changed forever
Three days of national mourning commence in Belgium today.
Yesterday, Prime Minister Charles Michel declared the move after 34 died and 200 more were wounded in three bombs at the Brussels airport and Malbeek Metro station.
Islamic State has claimed responsibility for the attack.
A minute's silence will be held in memory of the victims later.
Police are now hunting for a man seen on CCTV footage with two of the other bombers, shortly before the blasts at the airport. He is believed to be on the loose.
Hours after the explosions - investigators found a nail bomb during a raid in Brussels.
Last night Belgium's King Philippe addressed the nation - saying the 22nd of March will no longer be seen as an ordinary day:
"We give our support to the members of the emergency services and security.
Our gratitude also goes to all of those who gave their help spontaneously.
As to the threat we face, we will continue to respond with courage and dignity."
King Philippe addressed his people in French.
Last night, famous landmarks like the Eiffel Tower in Paris and Berlin's Brandenburg Gate were lit up in the colours of the Belgian flag.
And crowds gathered in a central square in Brussels to light candles in memory of the victims.
Authorities immediately launched raids around the city, during which they found items including a nail bomb and an Islamic State flag.
At home, flags will fly at half mast on government buildings today to mark both the atrocities in Brussels, and the recent drowning tragedy in Buncrana, Co Donegal.
President Michael D Higgins will sign a book of condolences at the Belgian embassy in Dublin.