Grieving Belgians held prayers in the rain in a central Brussels square carpeted with flowers and tributes to the 31 dead and 300 wounded in Tuesday's carnage, but there was also growing anger at the government for letting a string of militants slip through the net.
The raids came as under-fire Belgian investigators uncovered alarming new evidence of a European jihadist cell tied to bombings at Brussels' airport and metro, November's Paris attacks and a new French plot.
"Then, voices across Europe declared, 'Je suis Americain.' Now, we declare, 'Je suis Bruxellois' and 'Ik ben Brussel,' Kerry said in French and Flemish, the country's two main languages, after meeting Belgian Premier Charles Michel.
European authorities are under huge pressure to better coordinate the tracking of homegrown extremists and fighters returning from Syria, as evidence grows of a thriving jihadist network straddling France and Belgium.
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French police said they had foiled a terror strike by 34-year-old Reda Kriket -- a man previously convicted in Belgium in a terror case alongside Paris attacks ringleader Abdelhamid Abaaoud -- after arresting him and discovering explosives at his home.
French Interior Minister Bernard Cazeneuve said the arrest "foiled a planned attack in France, which was at an advanced stage."
Belgian police later arrested three people in connection with the new French conspiracy, prosecutors said.
In dramatic scenes, one of the suspects was shot in the leg at a tram stop in a huge operation by police in the Belgian capital's Schaerbeek district, where police this week found a bomb factory linked to the Brussels attacks.
German police said meanwhile they had arrested a 28-year-old Moroccan man whose mobile-phone text messages may link him to one of the Brussels bombers, the news weekly Der Spiegel and ARD TV channel reported.