A man has been arrested after being found in possession of a knife, a firearm and a quantity of powder, London's Metropolitan Police has said.br br Tesco Extra supermarket in Woolwich, southeast London, was evacuated on Saturday evening after reports of a gunman in the store with a knife and an explosive device.br br Video shows crowds gathering and a police cordon in place.br br Greenwich Metropolitan Police Service tweeted: "Man arrested after being found in possession of a knife, a firearm and a quantity of powder nr Tesco in General Gordon Sq #Woolwich #Greenwich just after 6pm today. No reports of any injuries. Man remains in custody and enquiries ongoing.