London's Metropolitan Police has confirmed a stabbing attack near London Bridge, saying it is treating the incident as "terror-related"."Police were called at 13:58 to a stabbing at premises near to London Bridge," the police force said on Twitter, adding it believes a number of people are injured. "A man has been detained by police."Police said the attack was being treated as "terror-related" as a precaution while the circumstances remain unclear, adding that they have shot one man.London Ambulance Service declared a "major incident", saying it had several crews at the scene.There was a heavy police presence and the area was sealed off as helicopters circled above. Several buildings were put on lockdown.Prime Minister Boris Johnson said: "I'm being kept updated on the incident at London Bridge and want to thank the police and all emergency services for their immediate response."Jeremy Corbyn, leader of the main opposition Labour Party, wrote on Twitter: "Shocking reports from London Bridge. My thoughts are with those caught up in the incident. Thank you to the police and emergency services who are responding."Social media footage showed police detaining a man, with gunshots heard in some videos. Other images showed people fleeing Borough Market.Connor Allen, an events manager who was near the scene, told Al Jazeera he heard "quite a few" gunshots. "Everyone just started running, that's what you do in that situation," he said, describing scenes of "pandemonium".In 2017, at least eight people were killed in the London Bridge area when attackers ploughed a van into pedestrians before carrying out a knife attack at Borough Market.