LONDON — Authorities said 21 people remained in critical condition Sunday after a rampage at the iconic London Bridge and a nearby market that left seven dead and dozens more injured.

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

"It is time to say enough is enough," Prime Minister Theresa May said Sunday. She called for tougher measures to contain Islamic extremism, saying 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."

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

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.

Twelve people were arrested Sunday in the East London community of Barking by counterterrorism police units, police said. More homes in the area were being searched.

Mark Rowley, head of counterterrorism for Metropolitan Police, said all the attackers are thought to be dead. He said eight officers fired about 50 shots, and that one member of the public suffered a gunshot wound. The injuries to that person were not believed to be critical, he said.

Rowley lauded the courage of officers who ran toward the attack as it unfolded.

"I am humbled by the bravery of an officer who will rush towards a potential suicide bomber thinking only of protecting others," he said.

Some of the 48 wounded taken to local hospitals were believed to be suffering serious and life threatening injuries, police said. Among them was a British Transport Police officer hospitalized in stable condition after showing "enormous courage in the face of danger, as did many others who were at the scene and rushed to help," Chief Constable Paul Crowther 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 at 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."

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

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.

Counter-terrorism special forces are seen at London Bridge 

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. Campaigning will resume Monday for the election Thursday.

Bacon reported from McLean, Va.