LONDON — Officers from the city's Terrorism Command arrested 12 people Sunday and intensive searches were continuing after a rampage at the iconic London Bridge and a nearby market left seven victims dead and dozens more injured.

Three suspected terrorists were killed in Saturday night's attack, the third in Britain in less than three months.

"The (Metropolitan Police Service) urges the public to remain calm but vigilant during this period," the service said in a statement. "If you see anything suspicious, no matter how insignificant you might think it is, please contact the anti-terrorism hotline."

Police said a van plowed into pedestrians on London Bridge, then drove to the nearby Borough Market where the assailants went on a stabbing rampage. Armed officers confronted three suspects, who police said were wearing "hoax" suicide vests, at the market. All three were shot dead.

Mark Rowley, head of counter-terrorism for Metropolitan Police, said all the attackers are thought to be dead, but investigators were working to round up potential accomplices.

Seven people died, in addition to the three attackers shot dead by police. Some of the 48 people taken to five local hospitals were believed to be suffering serious and life threatening injuries, police said.

Prime Minister Theresa May called for tougher measures to contain Islamic extremism in a statement outside her offices in Downing Street on Sunday. She said the recent attacks are not directly linked, but “terrorism breeds terrorism.”

“They are bound together by the single evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism,” she said. “It is an ideology that claims our Western values and freedom, democracy and human rights are incompatible with the religion of Islam."

"It is time to say enough is enough" May said.

London Mayor Sadiq Khan said he was "appalled and furious that these twisted and cowardly terrorists deliberately targeted innocent Londoners" and tourists. He urged all Londoners to "remain calm and vigilant today and over the days ahead."

President Trump tweeted solidarity with Britain, tweeting "WE ARE WITH YOU. GOD BLESS!" But he also took shots and political correctness and gun control.

“We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse,” Trump tweeted.

The secretary general of the Muslim Council of Britain, Harun Khan, also condemned the attack.

“Muslims everywhere are outraged and disgusted at these cowards who once again have destroyed the lives of our fellow Britons," he said "That this should happen in this month of Ramadan, when many Muslims were praying and fasting, only goes to show that these people respect neither life nor faith."

Daniel Ansah, 50, a security guard at Tito's restaurant in London Bridge, was present when the attack unfolded.

"It was horrific," he said. "I saw about three people running to the market and there were about five people on the floor."

He said he believed there were more than three attackers. Some of the wounded pleaded for help, Ansah said.

"Two men said 'help me,' they had gunshot or stab wounds on their backs and the blood was flowing," Ansah said. "It could have happened to me too. I'm traumatized."

French President Emmanuel Macron says French nationals were among the injured. He denounced the “abominable and cowardly” attack and said France will continue fighting “terrorism with all our strength alongside Britain and all other countries concerned.”

No group has claimed responsibility for the attack. Eyewitnesses said they heard the men shout "this is for Allah,” British media reported.

President Trump tweeted: "Whatever the United States can do to help out in London and the U. K., we will be there - WE ARE WITH YOU. GOD BLESS!"

Britain has weathered two other terror attacks in recent months. On March 22, five people were killed in London after Khalid Masood rammed his car into pedestrians on Westminster Bridge before fatally stabbing a policeman outside the Houses of Parliament. Masood was shot dead by police. The Islamic State said a "soldier" from the group carried out the attack. Police later said Masood had no links to extremist groups such as ISIS or al-Qaeda.

On May 22, Salman Abedi, a British-born suicide bomber, killed 22 people and injured 59 others at an Ariana Grande concert in Manchester, northwestern England. Abedi died at the scene. ISIS claimed responsibility but that claim cannot be verified.

The attack Saturday came on the eve of a benefit concert by Grande for victims of last month's Manchester Arena bombing in aid of the victims and their families. Police said the event would still take place, with additional security in place.

"We're deeply saddened to hear about last night's horrific attacks in London and our thoughts are with everyone affected," said Greater Manchester Police's Assistant Chief Constable Garry Shewan.

The ruling Conservative Party and the opposition Labour Party suspended election campaigning Sunday. it will resume on Monday.