London: British police have made seven arrests and searched six addresses after an attack near the British Parliament which left four people dead, including the attacker, and at least 29 injured, the country's top anti-terrorism officer said on Thursday. Police and MI5 were hunting for friends and associates of the attacker behind Wednesday's atrocity in Westminster, in a race to discover whether he was a 'lone wolf' or part of a broader terror cell. The chaotic incident saw the British Parliament on lockdown for hours and the heart of the British capital brought to a standstill. According to media reports, the attacker was already known to counter-terrorism officials. , On Thursday morning Assistant Commissioner of Police and Head of Counter-terrorism Mark Rowley revealed that police had raided six addresses and made seven arrests as part of their investigation, which covered London, Birmingham and other places. Hundreds of detectives had worked through the night on the case. He said police believed - which was being borne out by the continuing investigation, that the attacker "was acting alone and inspired by international terrorism". Police had no specific information about further threats to the public. However enquiries, searches and arrests were continuing, and investigators were still working to establish the attacker's motive, preparation and associates. He asked that the media not publish the name of the attacker at a "sensitive stage of the investigation". He also said seven people injured in the attack remained in hospital in a critical condition. Two members of the public died on Westminster Bridge as the attacker mounted the pavement at 2.20pm on Wednesday and sent tourists and locals "flying like footballs", as one eyewitness described it. The four people killed include a woman in her 40s, a man in his 50s, the police officer and the attacker, police confirmed on Thursday. Twenty-nine people were treated in hospital for injuries, including police officers. One woman fell into the Thames, she was later recovered and treated for serious injuries. The injured are believed to include five South Koreans, three French schoolchildren, and one German woman living in South Australia, who was treated in hospital for a foot injury. After mowing down pedestrians, the driver smashed into the railings of parliament and tried to enter. However not far inside the grounds he was confronted by an unarmed policeman, Keith Palmer, 48, who the attacker fatally stabbed. Armed officers then shot the intruder dead. A Conservative MP, Foreign Office minister Tobias Ellwood, tried to resuscitate the policeman. Mr Ellwood, a former army officer, lost his brother in the Bali attacks in 2002, was being hailed a hero after his selfless act in running towards the site of the attack to try to help the PC. Just hours after the terror attack on Westminster Bridge, police raided a flat in Birmingham - a move which several British media sources said was linked to the terror attack, though police declined to comment "for operational reasons". Dozens of police joined the raid, cordoning off several streets. According to reports, several men were taken into custody. BBC Newsnight said police were led to the address after tracing the rental of the Hyundai i40 car used in the attack. More police action is expected on Thursday, in a massive operation involving the police counter-terror command and intelligence service MI5. The murdered policeman was a husband and father, a member of the Parliamentary and Diplomatic Protection Service. He had been a police officer for 15 years. Assistant Commissioner Rowley said Mr Palmer "was someone who left for work today expecting to return home at the end of his shift, and he had every right to expect that would happen". He added that police were assuming the attacker's motive to be "Islamist related". Late in the evening British Prime Minister Theresa May gave a defiant speech outside Number 10 Downing Street, where she pledged life would go on in the capital in spite of the "sick and depraved terrorist attack". Parliament would meet "as normal" on Thursday, she said - though it is expected some normal business will be replaced by speeches addressing the attack. "We will come together as normal," Mrs May said. "And Londoners - and others from around the world who have come here to visit this great City - will get up and go about their day as normal. "They will board their trains, they will leave their hotels, they will walk these streets, they will live their lives. "And we will all move forward together. Never giving in to terror. And never allowing the voices of hate and evil to drive us apart." The Queen has cancelled a visit to New Scotland Yard which had been planned for Thursday. Extra police have been ordered onto the streets of London as a precaution, as a temporary measure. Westminster Tube station, the closest to the attack, is closed. The nation's terrorism threat level remains at severe - where it has been since mid-2014 - meaning another attack is "likely". There is only one higher level, when an attack is believed "imminent". She said it was "no accident" that the attacker had chosen Westminster, which she said was engrained with a spirit of freedom, democracy and human rights that made it a target for those who reject these values.