Two men are now in custody for possible roles in the bombing attack on a rush-hour subway train Friday that injured 29 people in London.
A senior British minister has renewed criticism of U.S. President Donald Trump for tweeting about the police inquiry into the bombing of a subway train in London.
"Severe still means that an attack is highly likely so I would urge everybody to continue to be vigilant but not alarmed".
The terror threat level in the United Kingdom has been reduced from "critical" to "severe" in the wake of the Parsons Green terror attack.
Police arrested an 18-year-old man earlier on Saturday following the attack in west London.
Ms Rudd's announcement comes just hours after police arrested a second man in connection with the bombing.
He is being held at a police station in south London under the Terrorism Act.
Some passengers suffered burns and others were hurt in a stampede to escape.
Police said on Sunday they were searching a residential property in Stanwell, Surrey, close to the perimeter of London's Heathrow Airport, in connection with the Hounslow arrest.
Responding to reported Isis claims that its followers have manufactured further explosive devices in London, Ms Rudd said: "They will always make claims that suit their intentions, but actually there is no evidence of that".
Prime Minister Theresa May also criticised Trump's comments about the attack as unhelpful. Soldiers and armed police were deployed to strategic locations such as nuclear power plants.