Paris police evacuated the Gare de l'Est train station today after a bomb threat, as authorities across Europe pressed on with efforts to prevent new violence after the worst terrorist attacks in decades.

The Paris prosecutor's office, meanwhile, said 10 people were arrested in anti-terrorism raids in the region, targeting people linked to a gunman who attacked a kosher supermarket and claimed ties to the Islamic State.

The developments, coming on the day that US secretary of state John Kerry arrived to "share a big hug with Paris", came a day after Belgian police killed two suspected terrorists in a firefight and arrested a third man.

A French police official said the Gare de l'Est station was closed "as a precaution". The station, one of several main stations in Paris, serves cities in eastern Paris and countries to the east.

In Berlin, police arrested two men Friday morning on suspicion of recruiting fighters for the so-called Islamic State group in Syria.

The Belgian raid on a former bakery was another palpable sign that terror had seeped deep into Europe's heartland as security forces struck against militants some of who may be returnees from Islamic holy war in Syria. Federal magistrate Eric Van der Sypt said returnees were an important part of the targeted searches.

Across Europe, anxiety has grown as the manhunt continues for potential accomplices of the three Paris terrorists, all of whom were shot dead by French police. Authorities in Belgium signalled they were ready for more trouble by raising the national terror alert level from two to three, the second-highest level.

France is on edge since last week's attacks, which began at the satirical newspaper Charlie Hebdo. The paper, repeatedly threatened for its caricatures of the Muslim Prophet Muhammad, buried several of its murdered staff members Thursday even as it reprinted another weekly issue with Muhammad on its cover.

Defence officials said France came under an unprecedented cyber assault with 19,000 cyber attacks launched after the country's bloodiest terrorist attacks in decades, frustrating authorities as they try to thwart repeat violence.

The attacks, mostly relatively minor denial-of-service attacks, hit sites as varied as military regiments to pizza shops but none appeared to have caused serious damage.

President Francois Hollande said France is "waging war" against terrorism and will not back down from international military operations against Islamic extremists.

He said: "It is not a war against religion, it's a war against hate."

He spoke to leading diplomats today as authorities remain on alert for new terrorist attacks. They are hunting for accomplices of three men claiming links to al Qaida and Islamic State who killed 17 people last week, before being killed by police.

He said France remains determined to continue its anti-terrorist operations in Iraq and northern Africa despite repeated threats of retaliation, saying: "We are not afraid."

Mr Hollande urged Europe to take tougher joint measures against terrorism.

Mr Kerry paid his respects in a show of American solidarity with the French people.

He met Mr Hollande and foreign minister Laurent Fabius before visiting the sites of the attacks, offering silent prayers and laying wreaths to honour the dead under heavy security.

After embracing Mr Hollande in the courtyard of the presidential palace, Mr Kerry said: "I think you know that you have the full and heartfelt condolences of the American people and I know you know that we share the pain and the horror of everything that you went through. Our hearts are with you."

Mr Hollande said the French people "were the victims of an exceptional terrorist attack. We must therefore together find the necessary response".

Mr Kerry and Mr Fabius laid a wreath at the Hyper Cacher kosher market, one of the two sites attacked last week. The wreath was the latest addition to a long stretch of flowers and candles placed alongside police barriers around the market.

At the market, they spoke briefly with Joel Mergui, the head of France's Rabbinical Council, before heading to the memorial to those killed at the Charlie Hebdo satirical newspaper by gunmen claiming to be acting on behalf of al Qaida's Yemen branch to avenge the publication of cartoons mocking the Prophet Mohammed.

There, at a makeshift memorial down the street from the newspaper's office, they laid another wreath and were joined by Francois Vauglin, the mayor of the district.

Mr Kerry, accompanied by US ambassador to France Jane Hartley, then walked the short distance to where a Paris police officer was killed by the gunmen and placed flowers at the site.

His visit came amid lingering criticism of the Obama administration's failure to send a cabinet-level official to Paris for last Sunday's unity march that attracted 40 world leaders and more than a million demonstrators.