The United Kingdom remained on high alert Saturday after the government raised the country's terrorism threat level to critical in a signal that another attack could be imminent.

London's Metropolitan Police arrested an 18-year-old man on Saturday in connection with the subway blast at the train station in Parsons Green in southwest London that left 30 people injured a day earlier.

Deputy Assistant Police Commissioner Neil Basu called the arrest "significant," but cautioned that the investigation into the attack was ongoing and that the threat level in the U.K. remains "critical."

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"This arrest will lead to more activity from our officers," he said. "For strong investigative reasons, we will not give any more details on the man we arrested at this stage."

The suspect was arrested in Dover – a major ferry port in southeast England – on Saturday morning, according to Metropolitan Police. He has not been charged with crime.

But it remained unclear on Saturday if they believe there is another suspect at large, and police urged people to remain "calm but alert."

U.K. officials on Friday raised the national threat level to "critical" after a bomb partially exploded aboard a train during the morning rush hour, injuring 30 and placing the country back on high alert after a series of devastating attacks earlier this year.

The Islamic State in Iraq and Syria (ISIS) said an affiliated unit had carried out the attack.

Friday's explosion was the latest attack to rattle Britain this year. In March, an attacker drove a car into pedestrians near Parliament, leaving six people dead and dozens injured.

In May, a suicide bomber killed 22 people at an Ariana Grande concert in Manchester. And in June, eight victims were killed when three attackers struck pedestrians on London Bridge before stabbing people at the nearby Borough Market.

Another attacker drove a van into Muslim worshippers near a mosque in Finsbury Park in north London.

The Manchester attack was the last time that the U.K. raised its threat level to critical, its highest level. It previously had not been triggered since 2007.