Councils, prisons, NHS trusts and schools will be required to take action to stop people being drawn into terrorism under new rules introduced today.
Laws passed earlier this year place public bodies under a statutory duty to identify and report those vulnerable to radicalisation.
The measure is part of the Government's One Nation attempt to tackle extremism.
It was set out in the Counter Terrorism and Security Act, which says: "A specified authority must, in the exercise of its functions, have due regard to the need to prevent people from being drawn into terrorism."
Security minister John Hayes said: "We have seen all too starkly and tragically the dangers of radicalisation and the devastating impact it can have on individuals, families and communities.
"The new Prevent duty is about protecting people from the poisonous and pernicious influence of extremist ideas that are used to legitimise terrorism.
"Protecting those who are vulnerable and at risk of radicalisation is a job for all of us.
"The new duty will make sure key bodies across the country play their part and work in partnership, as part of our One Nation approach to bring the country together to tackle extremism."
Guidance issued earlier this year by the Government on how the new duty should be implemented says:
:: Local authorities should consider whether publicly available IT equipment should use filtering to limit access to terrorist and extremist material and take steps to ensure children attending out of school settings are "properly safeguarded"
:: Schools will need to demonstrate they are protecting pupils from being drawn into terrorism by "having robust safeguarding policies in place to identify children at risk, and intervening as appropriate". They will also be expected to protect children from terrorist and extremist material when accessing the internet at school
:: Universities will be expected to carry out a risk assessment for their institution assessing where and how their students might be at risk of being drawn into terrorism
:: Healthcare workers should be trained to recognise signs of individuals being drawn into extremism
:: Prison bosses should carry out cell-sharing risk assessments for inmates
The duty applies in England, Wales and Scotland. The list of bodies subject to the obligation includes local authorities, NHS trusts, nurseries, schools, police, prisons and probation providers.
Universities and colleges will be included once further guidance on extremist speakers has been published.
Education Secretary Nicky Morgan said schools in England and Wales will receive guidance on identifying pupils in danger of radicalisation.
She suggested displaying an extreme intolerance of homosexuality was an example of the kind of behaviour that could cause concern.
"Sadly, Isis (Islamic State) are extremely intolerant of homosexuality," she told the BBC.
She added any action in those circumstances "would depend very much on the context of the discussion".
Ms Morgan said: "In this debate we need to have a very sensible, measured discussion. Schools are perfectly capable, they do this all the time.
"What this guidance is saying is that radicalisation ... is a very real threat in this country."
Christine Blower, general secretary of the National Union of Teachers, said the Government's Prevent programme is already "causing significant nervousness and confusion among teachers".
She added: "The jury is out as to whether extra statutory requirements are the most effective way to help young people stay safe, think critically, or reject engagement with groups who advocate violence.
"If pupil well-being and safety is the aim, the Prevent strategy is felt by many teachers to be counter-productive and wide of the mark.
"It risks closing down the very opportunities where the classroom can be used to develop democracy and explore human rights."
Sir Peter Fahy, National Police Chiefs' Council lead for Prevent, said: "We welcome the new statutory Prevent duties, ensuring more public bodies have a legal responsibility to help stop people being drawn into terrorism.
"Everyone has a role to play and partnership working is critical to the success of the Prevent initiative.
"We look forward to forging new partnerships with more agencies to safeguard and support vulnerable individuals to help achieve our shared aims of protecting the public from violent extremism and terrorism."
Councillor Ann Lucas, of the Local Government Association, said: "There is absolutely no place for extremism in our society and local government stands ready to play its part to help defeat it.
"Councils are already well placed in the heart of our communities to tackle extremism, build bridges between different groups and foster social cohesion.
"Extremism of any form allows intolerance and hate to develop and we must be on our guard to tackle those who promote extremist views and beliefs.
"Councils need to be resourced to undertake this extremely important work and are ready to support the new legislation.
"For some councils these new responsibilities will mean building on the significant work they have already done.
"For many others, where extremism is thankfully not an issue, it means analysing risks and introducing new measures to make sure it never develops."