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Six British men were ­arrested in Belgium ­yesterday after they were spotted in three old ­ambulances near Brussels while the city was in a fullscale terror alert.

A heavily armed anti-terror unit swooped on the group close to an Esso petrol station regularly used by Salah Abdeslam – the fugitive terrorist still on the run from the Paris massacres.

Six armoured trucks and a ­police car moved in after the ­converted vehicles parked up in a rest area beside the E411 ­highway in Bierges south-east of the capital around 2pm.

Last night the six Pakistani-born UK nationals were being ­questioned while officers ­continued to search the ­ambulances – two Fiats and a Mercedes with British plates.

Brussels remained in lockdown following an intelligence warning of an imminent Paris-style attack on the Belgian capital.

(Image: John Alevroyiannis)

One of the men held is ­understood to be known to British ­police who are assisting Belgium authorities.

Police are probing any link to Islamic State after using old ambulances was revealed to be a hallmark cover tactic used by the terror group.

Tanks and masked special ­forces squads were deployed to guard all major landmarks in the Brussels region while the manhunt ­continued for Abdeslam, 26.

The British men, who do not speak French, were said to be ­co-operating with detectives and have been fingerprinted to check for possible matches.

Specialist officers scouring the ambulances recovered clothes, food and money inside, raising suspicions they may have been heading for Syria.

(Image: Google)

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The threat level in Belgium was raised to the highest Level Four after an assessment by Coordination Unit for Threat Analysis (OCAM), meaning a threat is “serious and imminent”.

The vast underground metro network in Brussels was closed, train lines suspended and ­markets told not to open.

Main central shopping centres such as the upmarket Avenue Louise were ­deserted after the city’s mayor asked the major Apple store and a Marks & Spencer not to open and urged all cafes and ­restaurants to close at 6pm.

(Image: JOHN ALEVROYIANNIS) (Image: Getty Images)

Cinemas and railways ­stations were closed and planned football games from semi-professional to top-flight matches were called off as troops in balaclavas roamed the capital’s tense streets.

Top hotels such as the Radisson Blu also had armed guards outside. Belgium’s prime minister Charles Michel ­confirmed that the decision to raise the terror alert level in Brussels was based on “quite precise information about the risk of an attack like the one in Paris”.

The British Foreign Office warned UK nationals in the city to avoid ­crowded areas. Belgium’s Prime Minister said the fear was that ­several individuals with arms and explosives could launch an attack in several places.

(Image: JOHN ALEVROYIANNIS) (Image: REUTERS/Youssef Boudlal)

Mr Michel added: “We urge the public not to give in to panic, to stay calm. We have taken the measures that are necessary.”

The American Embassy in Brussels asked all US citizens in the country to stay at home. Anderlecht’s football match in the Belgian first ­division was ­postponed, with the league saying safety could not be guaranteed.

Gay nightclubs were warned not to open their doors last night and a concert by French singer Johnny Halliday,72, was called off in Brussels.

(Image: REUTERS/Francois Lenoir)

The city is home to the headquarters of the European Union, the Nato alliance and offices of many multinational ­corporations. The Belgian government’s crisis committee will meet again this ­afternoon to reassess the threat.

In Turkey, authorities said they detained a 26-year-old Belgian suspected of ­connections to Islamic extremists and ­possibly to the Paris attacks.

Ahmet Dahmani is suspected of having explored the areas in Paris that were ­targeted in the attacks that killed 130 and wounded 350.

(Image: JOHN ALEVROYIANNIS)

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Security officers continue to hunt for the suspected eighth gunman, Abdeslam, whose brother Ibrahim carried out the suicide bomb attack at the Comptoir Voltaire restaurant. The explosion killed only the terrorist.

A wanted poster showing Abdeslam how he might look disguised with a wig and glasses has been published.

He is ­understood to have been spotted in Brussels several times last week and police have a 24-hour hotline for people to call.

A worker at the Esso station at the centre of yesterday’s arrests said Abdeslam was a frequent customer there – “quite nice, smiling”.

A suspected terrorist plot linked to a converted ­ambulance was foiled in Hanover, Germany, on Wednesday night.

French intelligence officials are reported to have warned their German counterparts about a potential attack, sparking a string of security alerts linked to the vehicle ­spotted near the football stadium where the national team’s game against Holland was cancelled .

Speaking about ­yesterday’s arrests of Britons, a police ­commissioner in Belgium said: “Shortly before 14.00 hours, we found the ­presence of three old ­ambulances, now transformed into vans, on the highway rest area.

On board were persons of foreign origin, one of who is known to the UK police. These are British citizens of Pakistani origin. They do not speak French but they co-operate.”

Four men were arrested near the Jewish Museum in the city centre where a terror attack took place last year.

In France troops were guarding water facilities around Paris today as the fear of a chemical attack intensified.

The capital’s state-run water company, has increased the amount of chlorine added to supplies.

Fears of a chemical or germ attack rose after a dozen protective suits used to guard against viruses such as Ebola or chemical contamination have been stolen from the Necker hospital in Paris.