LONDON (AFP) - One of the main suspects in the brutal murder of a soldier in London was arrested in Kenya more than two years ago for seeking terror training, it emerged on Sunday, after police made more arrests.

Anti-terror investigators in France meanwhile were probing the stabbing of a soldier in Paris as he patrolled a busy shopping centre and transport hub on Saturday afternoon.

President Francois Hollande said no link to the grisly murder in London had been established "at this stage", but the French interior minister said there were similarities.

In Britain, Michael Adebolajo was pictured on the front page of several newspapers in the dock of a Kenyan court after he was arrested in 2010.

He was detained on the island of Pate, a few kilometres away from Lamu, which is a crossing point to Somalia, according to the Sunday Telegraph and Independent on Sunday.

The reports said Adebolajo, 28, had been accused of trying to lead a group of youths trying to join Somalia's Al-Qaeda-linked Shebab movement.

He was released and deported to Britain after appearing in court in Mombasa with six other men in November 2010.

The disclosure raises fresh questions about the monitoring of Adebolajo and the other suspect in the murder, 22-year-old Michael Adebowale, by Britain's intelligence services.

Adebolajo was captured on video carrying bloodied knives and a meat cleaver after Wednesday's attack saying he had killed off-duty soldier Lee Rigby because British troops were killing Muslims.

He and Adebowale remain under armed guard in a stable condition in separate hospitals after they were shot by police at the scene. They are not yet strong enough to be questioned by detectives.

It also emerged that Adebowale was arrested in London two months ago after local traders complained about a group of Muslim activists.

In Britain, armed police arrested three more men on Saturday on suspicion of conspiracy to murder.

Two men aged 28 and 24 were arrested at a home in southeast London. Police used 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.

Officers also searched four properties in London.

Both the suspects in the murder are Muslim converts from Nigerian Christian families, raising concerns in Britain about the radicalisation of young men.

Home Secretary Theresa May, the interior minister, warned there were "potentially" thousands of people at risk of being radicalised as she indicated plans for a fresh crackdown on extremist groups.

May said around 500 police and intelligence officers were working on the "horrific murder" of the soldier.

But she said "all the indications" pointed to a lone wolf-style incident rather than a wider planned operation.

In Paris, counter-terrorism investigators were handling the probe into the stabbing of soldier Cedric Cordier as he patrolled the La Defense district.

President Hollande said authorities were still piecing together information on the bearded attacker, who melted into the crowd without a word after the stabbing.

"We still do not know the exact circumstances of the attack or the identity of the attacker, but we are looking at all options," Hollande said during a trip to Ethiopia.

Hollande cautioned against drawing a link to the London killing, but Interior Minister Manuel 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." The stabbing was captured by surveillance cameras.

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

"It's a hugely worrying development," director Fiyaz Mughal told AFP, saying the organisation had been informed of 162 incidents in a 48-hour period, compared to a normal daily average of up 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.