in the case of an attack with melee weapons such as axes or knives you need to scatter. go away. slower than the attackers? still go. you're in a target rich environment. if your directing is rolling, tottering, hobbling away you stand a very good chance of getting away. negotiation or surrender in these situations are not options. if in a life or death situation don't hesitate to put anything edged or your thumbs into their eyes.
i'm telling you this because all you can expect from the police and other authorities is for them to cover their arses.
in the less likely event you spot what you think is a bomb, inform staff. don't operate your phone. don't photograph. light sensitive and radio wave sensitive devices exist. inform staff. you should be relatively safe from a backpack sized bomb at more than 50m away and behind hard cover.
in the even less likely event you spot and actual device memorise everything you can, don't photograph it. go report it. and then you may as well go, the area is about to be cleared by police and private security in no time anyway. you will be asked under pressure for anything about the device. size, smell, wires, audible ticking, switches, batteries, tape messages are all useful information.