Theresa May has outlined new plans to prevent hundreds of young British Muslims from travelling to Iraq and Syria to fight with Islamic State militants.
The Home Secretary told the Conservative Party conference in Birmingham: "There is a battle raging in the Middle East for the heart and soul of Islam.
"We must do everything to defeat this ideology and prevent the radicalisation of young British Muslims ."
However, her proposals to curb the activities of radical Islamist preachers even if they have committed no crime have been criticised as "wholly wrong" by civil liberties campaigners.
Mrs May said the threat posed by IS was made greater by the fact that at least 500 British nationals were in Iraq and Syria, many of them to fight.
The Cabinet minister said she had the power to keep those with dual nationalities, and naturalised British citizens, out of Britain, and to strip those planning to travel to Syria and Iraq of their British passports.
She revealed she had already used this power 25 times in relation to Syria.
The Home Secretary said under a new Counter-Terrorism Bill to be introduced by the end of the November, police will be given new powers to seize the passports of suspects.
A new criminal offence - to prepare and train for terrorism overseas - would also be created.
She told delegates the Home Office would drive through a new cross-governmental counter extremism strategy.
Outlining proposals that will feature in the Conservative manifesto, she advocated new Banning Orders for extremist groups that currently fall short of the threshold for proscription.
They would apply to groups deemed to be inciting hatred or promoting the overthrow of democracy. The plans would also make it a criminal offence to become a member of, or raise money for, the groups concerned.
She also announced plans for Extremism Disruption Orders to restrict the movement of extremists who stay within the law, "but still spread poisonous hatred".
Individuals who are deemed to be extremists but are not guilty of terrorist offences could also face other controls if the Conservatives come to power, which could stop them from associating with named individuals, speaking at public meetings, using social media or broadcasting.
Mrs May said IS, formerly known as ISIL, had "made clear their ambitions and they have made us their enemies, and the lesson of history tells us that when our enemies say they want to attack us they mean it".
"We must not flinch... we must act to destroy ISIL," she said.
Emma Carr, director of Big Brother Watch, a civil liberties think tank, said of the proposals: "In a democratic country, it is wholly wrong for people to be labelled an 'extremist' and face having major restrictions placed on their freedom without facing a due legal process and a transparent and accountable system.
"The Home Secretary must think very carefully about the international precedent that this policy would set and consider the potential consequences for members of the public."