LONDON (AFP) - Four men were arrested in London on Tuesday on suspicion of terrorism-related offences by officers investigating Islamic extremism, Scotland Yard said.

The men, aged 20 and 21, were detained "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism" and are being held for questioning. One of the men, aged 21, was stunned with a Taser gun during his arrest although he was not injured, police said.

Unusually for Britain where most officers do not carry guns, armed police also attended one of the arrests although no shots were fired. Police are now searching an undisclosed number of addresses and vehicles in west and central London.

British Prime Minister David Cameron last month outlined tougher measures which he wants to impose on Britons planning to travel and fight with ISIS (Islamic State in Iraq and Syria), plus those who could return to launch attacks on home soil.

Some 500 British extremists are estimated to be fighting in the two countries. British police made 69 arrests linked to fighting in Syria in the first half of 2014, a rate five times higher than in 2013.