London commuters are beginning their working week with heavy security on transport routes following last week's terror attack.
An improvised device exploded on a London Underground train as it arrived at Parsons Green station during the Friday morning rush hour, injuring 30 passengers.
Two men have been arrested in connection with the incident.
An increased number of police officers could be seen on British streets over the weekend, with police saying there was an emphasis on "crowded places, iconic sites, transport hubs and ports."
On Sunday, Home Secretary Amber Rudd said the UK's terror threat level had been downgraded from "critical" back to "severe," meaning an attack is now considered highly likely rather than imminent.
"Members of the public will be wondering what that means in terms of their travel into work tomorrow," Metropolitan Police Assistant Commissioner Mark Rowley said in a Sunday statement following the announcement.
"For practical and precautionary reasons we made the decision that the increased resources will continue for the beginning of this week. So the public will still see that high level of policing presence; some armed, some unarmed," he said.
Rowley said progress in the investigation into Friday's attack had allowed the Joint Terrorism Analysis Center -- the independent body that assesses threat -- to lower the terror threat level, and that military support would be phased out over the week.
Detectives from the London Metropolitan Police Counter Terrorism Command arrested a 21-year-old man late Saturday in Hounslow, west London, in connection with the bombing.
"The man was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody," police said in a statement Sunday.
Police had previously arrested an 18-year-old suspect in the departure area of the port of Dover on Saturday. Dover is major port town about 80 miles southeast of London.
The teen was taken into custody "on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism," authorities said.
The arrests are part of a major manhunt for the perpetrators of Friday's attack.
By Saturday afternoon, detectives had spoken to 100 witnesses and were combing through hours of CCTV footage as well as videos and photographs sent in by the public, Neil Basu, the senior national coordinator for counterterrorism policing, said in a statement.
"The public's help remains crucial, and I urge anyone with information, no matter how significant they think it is, to continue letting us know," he said.
Police searched properties in Stanwell and Sunbury-on-Thames Sunday.
Shortly after the explosion, ISIS claimed involvement via its Amaq News Agency.
But when asked if ISIS may have had a hand in the attack, Rudd said: "It is inevitable that so-called Islamic State or Daesh will reach in and try and claim responsibility. We have no evidence to suggest that yet."
Rudd said the government is releasing an additional 24 million ($32.6 million) for counterterrorism operations around the country, "particularly to support operations where they are protecting people in public places."