London - British police warned on Friday they were facing an "increased level of terrorist activity", the day after a knifeman was arrested near parliament and a woman was shot in a separate police raid.Just weeks after the attack on the Houses of Parliament that left five people dead and scores injured, London is once again on high alert as Britain prepares for a snap election on June 8."Yesterday was an extraordinary day in London," Deputy Assistant Commissioner Neil Basu said, after confirming six arrests overnight."I wanted to reassure the public that this increased level of terrorist activity is being matched by our action... We are making arrests on a near daily basis."A 27-year-old man remains in custody after being arrested near parliament on suspicion of terrorism offences and possession of knives, in what police confirmed had been an ongoing investigation.Just hours later, armed police raided a property in north London as part of an unrelated counter-terrorism investigation, firing CS gas as they entered and shooting a woman in her 20s.Six people have been arrested as part of the probe, while the woman - who is also suspected of involvement - remains in a serious but stable condition in hospital. "Due to these arrests that have been made yesterday, in both cases I believe we have contained the threats that they posed," Basu said.Suspect 'tracked to Westminster'Britain's national terror threat level has been at "severe", meaning an attack is highly likely, since August 2014 - and remained unchanged after the attack on parliament on March 22.Khalid Masood drove a car through pedestrians on Westminster Bridge before crashing into the gates of parliament. He ran inside and knifed to death a policeman, before being shot.Police have said they may never know why he did it.The man arrested on Thursday, metres from parliament and Prime Minister Theresa May's Downing Street residence, had reportedly been tracked by police and the domestic intelligence agency MI5.Newspaper reports said he had been followed as he travelled into Westminster, in an investigation that originally began with a tip-off by someone close to him."They stopped and searched him as part of an ongoing counter-terrorism investigation," Basu said, without giving further details.An AFP photographer saw firearms officers surrounding the bearded man and pinning him to the ground, before putting him in handcuffs.Construction worker David Wisniowski, who was working on a building site next to the incident, told AFP he saw "three knives on the floor, one big one and two small".'Debt of gratitude'Hours later in Harlesden, a suburb in north London, armed police launched an unrelated counter-terrorism operation that resulted in six arrests overnight, five in the area and one in Kent, southeast England.The address had been under observation and the woman who was shot was one of the subjects of the investigation, Basu said."Her condition is serious but stable. Because of her condition she has not yet been arrested. We are monitoring her condition closely," he said, adding that the police watchdog had been informed.During an election campaign speech on Thursday evening, Prime Minister Theresa May paid tribute to the police and security services, saying they were owed "a huge debt of gratitude".Terrorism has yet to feature heavily in the campaign for the June 8 vote, although May's Conservatives have sought to exploit the anti-nuclear stance of opposition Labour leader Jeremy Corbyn.Foreign Secretary Boris Johnson claimed the leftist leader "seems to have no grasp of the need for this country to be strong in the world".