Home Secretary Theresa May has defended the UK's security measures in her first major speech on counter-terrorism.
She also said that British extremists had already travelled to Somalia to train and fight.
If UK security services did not intervene, then these individuals would return home to the UK to attempt to commit mass murder, Mrs May added.
She promised to do "absolutely nothing which will put at risk Britain's national security".
And she said core counter-terrorist policing capabilities would be maintained.
Spending on counter-terrorism "will remain high", she added, with more than £2bn for policing alone being spent in the next four years.
She also said that an alleged al-Qaeda member from the Arabian Peninsula was arrested in the UK on suspicion of planning a terrorist attack in this country earlier this year.
The home secretary said: "We know that people from this country have already gone to Somalia to fight.
"It seems highly likely, given experience elsewhere, that if left to their own devices we would eventually see British extremists, trained and hardened on the streets of Mogadishu returning to the UK and seeking to commit mass murder on the streets of London."
She also stressed that MPs like her, who had been on the front benches of government or opposition for the last 10 years, "will have thought long and hard about issues such as control orders, pre-charge detention and other counter-terrorism powers".
"In Parliament we have debated these issues, considered the balance between liberty and security and voted on the legislation," Mrs May added.
"I don't believe the previous government got the balance right but let me make clear: I will do absolutely nothing which will put at risk Britain's national security.
"So where necessary we will enhance our protective security measures. We will invest in conflict prevention and stopping terrorist plots overseas, we will refocus the strategy for preventing radicalisation in the UK, and we will strike a better balance between our liberties and our security."
Ms May also said she would not create a database enabling the state to listen in on every private conversation in the country.
"I want an approach which is more targeted against extremist individuals, but that impacts much less on the good people of our communities.
"I want an approach which allows people to enjoy their liberty in safety and security. And I want an approach that is effective in dealing with an evolving threat. That is what we will deliver."