Police questioned seven terror suspects in Belgium and France on Friday as US Secretary of State John Kerry declared "Je Suis Bruxellois" during a solidarity visit after bombings in Brussels claimed by the Islamic State group.
Grieving Belgians were holding prayers in the rain in a central Brussels square carpeted with flowers and tributes to the 31 killed and 300 injured by the airport and metro suicide blasts on Tuesday.
The under-fire Belgian police arrested six people in raids linked to the Brussels carnage, while in France, police said they had foiled a terror strike by a man linked to the Belgian ringleader behind the November attacks on the French capital.
The Belgian government faces heavy criticism over how the Brussels attackers slipped through the net, with three of them known to police and said to have links to the November Paris attacks in which 130 people died.
As US officials said two Americans were among the Brussels dead, Kerry said he stood by the Belgian people, echoing their backing for the United States after the 9/11 terror attacks.
"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.
The phrasing recalled former US president John F. Kennedy's famous "I am a Berliner" speech at the height of the Cold War, as well as the "Je suis Charlie" outpouring of solidarity after last year's attacks on French satirical magazine Charlie Hebdo.
In the stunned city, home to the European Union and NATO, mourners returned Friday to the Place de la Bourse square where they stood silently under umbrellas, some in tears and others still in shock.
Their grief mixed with growing anger about the apparent failure of the Belgian authorities to stop the bombers before they struck.
"The government knows a lot but they do nothing. Why didn't they do something to stop this attack? I think the government is a bit to blame for this situation," said Sergio Jorge de Oliveira Silva Lima, 38, a Portuguese citizen who has lived in Belgium for 15 years.
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.
French officials said they had arrested 34-year-old Reda Kriket late Thursday near Paris, uncovering a small quantity of explosives at his home.
Kriket was convicted in absentia last year in Belgium alongside Abdelhamid Abaaoud, the suspected Paris attacks ringleader killed by police shortly after the carnage in the French capital.
French Interior Minister Bernard Cazeneuve said the arrest "foiled a planned attack in France, which was at an advanced stage."
Belgian officials meanwhile said a series of raids in the capital Thursday yielded six arrests -- included three people detained "outside the door of the federal prosecutor's office".
A huge manhunt is underway for at least two suspects -- one of the airport attackers wearing a hat whose bomb failed to go off and another man seen in the metro with the bomber there.
Many of the Brussels assailants have links to Paris.
Prosecutors have confirmed that Khalid El Bakraoui, who blew himself up at Maalbeek metro station shortly after his brother Ibrahim did the same at Zaventem airport, was the subject of an international terrorism warrant over the Paris attacks.
Investigators also say he rented an apartment in Brussels used by key Paris suspect Salah Abdeslam, who was arrested in the Belgian capital last week, and a flat in the city of Charleroi where the Paris cell plotted the attacks.
The Belgian government has also admitted "errors" and two ministers offered to resign after Turkey said Ibrahim El Bakraoui had been arrested and deported and that Belgium had ignored warnings that he was a "foreign terrorist fighter."
The brothers were also listed in American terrorism databases, television network NBC reported.
Abdeslam's lawyer Sven Mary said Thursday his client "didn't know" in advance about the Brussels attacks, and would no longer fight extradition to France.
Belgium has lowered its terror alert to the second-highest level for the first time since the attacks, but the police and military presence on the streets of the capital remains high.
Harrowing stories continued to emerge from survivors of the attacks, in which people of around 40 nationalities were killed or wounded, testament to the cosmopolitan nature of Europe's symbolic capital.
Brazilian professional basketball player Sebastien Bellin said that as he lay bleeding profusely and fearing death at Brussels airport, he thought of something odd: his daughter's tennis skills.
"I just didn't want my girls to grow up without a dad, you know?" Bellin, 37, told US network ABC from his hospital bed.
An Indian flight attendant who was pictured covered in dust and blood, making newspaper front pages around the world, remains under sedation in hospital, her employer said Friday.
Indian airline Jet Airways told AFP that 40-year-old Nidhi Chaphekar, who is from Mumbai, "is in a stable condition and not in a coma. She is resting and under sedation for her comfort.
Among only three fatalities formally named so far are Peruvian Adelma Marina Tapia Ruiz, 37. Her husband Christophe Delcambe, and their three-year-old twin daughters, only survived because the girls had run off and their father had chased after them. (AFP)