LONDON (CNN) -

The United Kingdom may have raised its terror threat level, but the streets of London don't appear to be fazed.

Londoners and tourists did not let the announcement Friday affect their weekend plans. Public transportation hubs were bustling, tourists attractions were crowded and visible police presence was low.

"If you let things like that rule your life, you'd never do anything. So although you're aware of it, you hope that the government is on top of the situation," said Carl Toon, passing through King's Cross station with his family.

Jodie O'Rourke, an events manager in London, echoed that sentiment.

"You don't think every time you get on the tube there's anything going to happen," she said.

In response to events in Iraq and Syria, where ISIS militants have seized a large swath of territory, the UK government increased the country's threat level from "substantial" to "severe," the fourth highest of five levels.

"That means that a terrorist attack is highly likely, but there is no intelligence to suggest that an attack is imminent," Home Secretary Theresa May said.

While it's been widely reviled internationally, ISIS has managed to attract some support among Muslims and has drawn foreign fighters.

Even without specific threats in the West, ISIS' track record in Syria and Iraq -- where it was known to massacre minorities, forcefully institute Sharia law and stage executions and stonings -- suggest it may be capable of anything.

Prime Minister David Cameron said the group poses a "greater and deeper" threat than Britain has ever known.

"This is al Qaeda version 6.0," Ryan Crocker, a former U.S. ambassador to Iraq and Syria, told CNN on Friday. "They are like nothing we have ever seen before."

Fears of a Muslim witch hunt in the UK?

But not all are buying the government's dire predictions.

"I think it's just trying to make people scared really, that's all it is, said Dill Shawki, a Ph.D student in London. "Make people scared and they'll be more likely to support something if the government then eventually did try to go into Iraq."

Cameron's pledge to crack down on radical Islam is leaving some feeling uneasy, fearing that it could turn into a witch hunt against all Muslims in the UK.

"Being a Muslim is synonymous with being a terrorist nowadays," said Antara Muhajimi, wearing a full face veil in London's East End. "People are scared of what they don't know most of the time," she said, "They attack [Islam] because they don't know it. It's just part of human nature."

"Adhering to British values is not an option or a choice; it is a duty for those who live in these islands," said Cameron. "And in the end it is only by standing up for these values that will defeat the extremism, protect our way of life and keep all our people safe."

"Regarding these 'British values,' Muhajimi said, "I wouldn't know what they are, I only know what my values are, and how to treat other people with them.

"Kindness and patience and acceptance, that's what's going to keep a community together," she said.

Travel restrictions, an increase in police activity

Cameron said military force is among the tools that can be used against ISIS, while adding that aid, diplomacy and political influence should also be part of Britain's response.

That said, his focus Friday wasn't so much about what to do about ISIS overseas as it was keeping British citizens back home safe.

The Prime Minister vowed he will soon announce plans to stop would-be jihadists from traveling to Syria and Iraq and to make it easier to take their passports away.

Britain also needs to do more to stop current fighters from returning from the Middle East and to deal decisively with those who already have returned, he said.

UK authorities estimate that 500 Britons have gone to Syria and Iraq to fight with Islamist groups.