PARIS (AFP) - French anti-terror investigators on Sunday probed the stabbing of a soldier in Paris in an attack that echoed the grisly killing of another soldier in London, where British police were holding three new suspects.

French soldier Cedric Cordier was in hospital in a stable condition after the Saturday stabbing in a busy underground shopping and transport hub where he had been on patrol with two colleagues.

French President Francois Hollande said the stabbing could not be linked to the London murder "at this stage", although his interior minister, Manuel Valls, said the "sudden violence of the attack" was similar.

The assault on another soldier three days after Afghanistan veteran Drummer Lee Rigby was hacked to death on a London street in an Islamist attack will raise fears of a spiral of brazen violence against Western soldiers on their home soil.

In Britain, three men were arrested on Saturday on suspicion of conspiracy to murder Mr Rigby.

Two men aged 28 and 24 were arrested at a home in southeast London. Police fired a Taser electric stun gun on the older suspect, and on a 21-year-old man they arrested in a street around a mile from the murder scene.

All three are being held by detectives from the Counter Terrorism Command supported by specialist firearms officers.

Officers also searched four residential addresses in southeast London.

The two men suspected of murdering Mr Rigby, 28-year-old Michael Adebolajo and 22-year-old Michael Adebowale, remain in a stable condition under armed guard in separate hospitals after being shot by police at the scene of the killing.

After the killing, the pair launched into an Islamist tirade against British military involvement in Muslim countries, captured on film by a passerby whose footage of Adebolajo, hands bloody and wielding a knife and meat cleaver, shocked the nation.

Both suspects were brought up by Nigerian Christians and converted to Islam in their teens, and recently were seen handing out extremist literature in the streets - to the concern of their families.

President Hollande said French authorities are still piecing together information on the Paris stabber, who staged an equally public attack but then melted into the crowd without a word.

"We still do not know the exact circumstances of the attack or the identity of the attacker, but we are looking at all options," Mr Hollande told reporters accompanying him on a trip to Ethiopia.

Mr Hollande cautioned against drawing a link to the London killing, but Mr Valls said: "There are elements, the sudden violence of the attack, that could lead one to think there could be a comparison with what happened in London."

French Defence Minister Jean-Yves Le Drian said Mr Cordier had been targeted because he was a soldier and said he was working with Mr Valls to crack down on "terrorism".

"We are pursuing a merciless fight against terrorism and against any act that threatens our security," he told reporters after visiting the wounded man in hospital.

The local prosecutor's office said an anti-terror unit would handle the investigation of the stabbing, which was captured by surveillance cameras.

The attacker, described by police as bearded and around 1.9 metres tall, approached the 23-year-old soldier, stabbed him and then disappeared into the crowd.

Mr Cordier, who was armed and in uniform, was patrolling in Paris's La Defense business district as part of France's Vigipirate anti-terrorist surveillance scheme, which deploys troops at high-profile tourist, business and transport sites.

In Britain, Faith Matters, a state-funded organisation which works to reduce extremism, said it has recorded a huge increase in anti-Muslim incidents reported to its helpline since the killing.

"It's a hugely worrying development," director Fiyaz Mughal told AFP, saying the organisation had been informed of 162 incidents in the past 48 hours, compared to a daily average of four to six.

They were mainly verbal attacks on women wearing the Islamic headscarf in the street, he said, but there were also online attacks and some violence.

A number of people have been charged over malicious comments made on social networking sites.