London: ISIS claimed involvement in the London tube train blast but has offered no evidence to support the claim.
The station in Parsons Green, southwest London, where an improvised device exploded as a train stopped during the Friday morning rush hour, reopened early Saturday -- with a visible police presence as doors first opened.
Meanwhile, hundreds of police officers scoured CCTV footage, questioned dozens of witnesses and studied the remnants of the device in search of clues to the identity of those responsible.
Prime Minister Theresa May announced late on Friday that the UK terror threat level had been raised to its highest level.
"The public will see more armed police on the transport network and on our streets, providing extra protection. This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses," May said.
"The threat of terrorism that we face is severe but together, by working together, we will defeat them," she said.
The government's emergency COBRA committee, chaired by UK Home Secretary Amber Rudd,
will meet Saturday afternoon, a government spokesman said. It will be the second COBRA meeting since Friday's attack, bringing together the country's police and intelligence chiefs.
Investigators were making "excellent progress" toward identifying, locating and arresting those behind what police have labeled a terrorist attack, Mark Rowley, assistant commissioner of the Met Police, said late Friday.
Asked about possible ISIS involvement, Rowley told reporters it is "routine" for the extremist group to take responsibility for attacks in "these sorts of circumstances," regardless of its actual involvement.
London Mayor Sadiq Khan welcomed news of the arrest, saying "significant activity" could be expected Saturday and over the days ahead. He also urged anyone with information related to the investigation to contact police.
"It is important we all remain alert and vigilant. There will be an increased police presence across London today and over this weekend, including additional armed officers. They are there to keep us safe," he said.
"London will never be intimidated by terrorism. We will always defeat those who seek to harm us and destroy our way of life."
The British Transport Police tweeted: "Nationwide, extra officers from BTP are on duty today. If you see something suspicious, don't delay."
Friday's attack was the fifth terrorism incident in the UK this year. But it's the only one in 2017 in which nobody has died. The previous four saw 36 people killed.