Six people arrested in Brussels police operation after attacks: prosecutors

It said the arrests were made during police searches in the Brussels boroughs of Schaerbeek in the north and Jette in the west, as well as in the centre of Brussels itself.

It is understood more than 50 residents, including many children, are among those evacuated.

Police raids have discovered an ISIL flag, chemicals and a nail bomb at a house in Brussels hours after explosions devastated the European city.

At least one other man seen with them on airport security cameras is on the run and a fifth suspected bomber filmed in the metro attack may be dead or alive.

"The arrest is the result intricate work carried out over the last few weeks by intelligence services", said Mr Cazeneuve.

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The bombings came barely four months after suicide attackers based in Brussels' Molenbeek district slaughtered 130 people at Paris nightspots, and intelligence agencies had warned for months a follow-up strike was inevitable.

"Our freedom was struck in the heart this morning in Brussels, as it has been before in Paris, London and Madrid, " he said.

Brussels Mayor Yvan Mayeur said 20 people died and more than 100 were wounded in the subway blast.

- Experts said attack in Belgium is not a surprise. Obama, speaking in Havana, Cuba, said that the USA stands in solidarity with the people of Belgium.