David Cameron said extra security measures are now being enforced after the UK's terror threat level was raised to severe.
But he said people "should continue to go about our daily lives in a normal way".
He said the British public may see some changes with policing and the number of armed police.
Earlier he warned that the Islamic State poses a "greater and deeper threat to our security than we have known before".
The PM wants to confiscate passports. But in Cardiff they want to solve extremism at it's roots - they just can't get the help to do it.
David Cameron has unveiled a series of new measures to combat the threat of terrorism.
Rumours suggesting London's Tube network will be targeted in a terrorist attack on Monday are "completely untrue", police say.