Three more men arrested over London train bomb attack

British police arrested three men in south Wales over last week's bombing of a busy commuter train in west London, meaning five people are now being questioned by detectives over the attack which injured 30 people.

A 25-year-old man was arrested on Tuesday evening in Newport, while two others, aged 48 and 30, were detained at another address in the Welsh town in the early hours of Wednesday, London police said.

The arrest comes as Britain remains on high alert following Friday's Tube attack. London's financial district was on lockdown on Wednesday morning following reports of a suspicious package.

Pictures from the scene showed police vehicles and scores of officers swarming the area after the package was first reported just before 10am. A City of London Police spokesperson later said its was deemed to be non-suspicious.

The home-made bomb went off on Friday during the morning rush hour on a packed train at Parsons Green underground station, engulfing the carriage in flames, although it appeared that the device did not fully explode.

"This continues to be a fast-moving investigation," said Commander Dean Haydon, head of London police's Counter Terrorism Command.

"A significant amount of activity has taken place since the attack on Friday. We now have five men in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack."

In the aftermath, the authorities raised Britain's threat level to its highest rank of critical, meaning an attack was considered imminent, but lowered this after arresting two men on Saturday.

An 18-year-old was arrested in the departure lounge at the port of Dover on Saturday and another suspect, 21, was detained hours later in the west London suburb of Hounslow.

Islamic State militants claimed responsibility although both British and US officials have cast doubt on the statement, saying there was no evidence any recognised militant group had ordered or organised the bombing.

Friday's bombing was the fifth major attack regarded by authorities as a terrorist incident in Britain this year which have claimed the lives of 36 people.