Police sealed off the Tesco Extra supermarket in Woolwich, southeast London, on Saturday evening (April 25) after a major incident at the store caused it to be evacuated.
Met Police have said that a man was searched by officers and a firearm & a quantity of white powder was found.
Some eyewitnesses have claimed on social media that the man in question also had a knife and an explosive device.
At least one EOD (explosive ordnance disposal) officer can be seen inside the police cordon.
Greenwich and Woolwich MP Matthew Pennycook has tweeted he is "aware of a serious incident" and nobody was injured.