The fight against the threat of terrorism is complex. It involves many different arms of the state, from ordinary police walking the beat to MI5 operatives working deep undercover.
Since 11 September 2001, the government has radically changed how the UK deals with terrorist threats, with a brand new terrorism law and increased spending on anti-terrorism measures.
A terrorist suspect can obviously be held if police are pursuing them for a specific crime such as conspiracy, or murder. But the police can also draw on specific anti-terrorism powers.
The Terrorism Act 2000 is the principal piece of legislation at the authorities' disposal. Its measures include:
New crimes including inciting a terrorist act and providing facilities or support to terrorist networks at home or abroad.
Shortly after the 11 September attacks, Parliament passed a second bill, prepared in the wake of the atrocity.
The Anti-Terrorism Crime and Security Act 2001 (ATSCA) is a complex but short bill which is designed to prevent terrorist suspects from using the immigration system to prevent their detention or removal.
It also included new measures to freeze of suspected terrorist assets.
So how do these acts work?
Initially the most controversial element of the Terrorism Act was that it widened the definition of terrorism, something that human rights campaigners claimed would lead to poor policing if investigations were not properly focused.
The Terrorism Act allowed the police to hold and question a suspect for up to seven days without charge, subject to court scrutiny.
RECENT ANTI-TERROR SPENDING
£85.5m to NHS bioterrorism project
£49m to Metropolitan Police
£15m to Special Branch operations
Source: Home Office
That has now been extended to 14 days, although there are restrictions on the circumstances in which the police would be granted the full period.
People held under the Terrorism Act do not necessary face charges under it, they could go on to face a criminal charge under any other piece of legislation.
What are the figures for Terrorism Act arrests?
According to the latest available figures from the Home Office, 701 people were arrested between 11 September 2001 and 31 December 2004.
Of those, 119 people have been charged with terrorist-related offences and 17 people have been convicted.
A further 135 have been charged under other legislation, such as that dealing with fraud, immigration breaches or firearms and explosives offences.
Some 59 of those arrested were handed over to immigration officials for breaches of their right to remain in the UK. Another 22 people are on police bail, pending further inquiries.
The Association of Chief Police Officers says about a quarter of all the arrests were related to Northern Ireland's paramilitary organisations, but it keeps no detailed records of the ethnicity or religion of the others arrested.
Of those who were arrested but then released, police chiefs say these were "short-term detentions for routine inquiries while passing through ports".
And what about ATSCA?
Although its name is not well known, the law is behind the most controversial of anti-terror detentions. The Home Secretary can use its powers to take and indefinitely detain a foreign terrorist suspect.
The system was introduced because the security services believed there were a small number of individuals in the UK who posed a risk to public safety - but against whom there was insufficient evidence to bring a full criminal trial.
But, to complicate matters, ministers said these men could not be deported because they have an obligation under international human rights conventions not to send someone to a country where the individual's life would be endangered.
ATSCA created a system of allowing initially 17 men to be held until they were either considered suitable for release or chose to voluntarily leave the UK. Eleven remain in detention.
Campaigners dub this the "British Guantanamo Bay" system - a charge the government vehemently denies.
So are these laws working?
The government believes they are the best available measures for the security services, but warns against complacency. A number of major terrorism-related trials are due to come before the courts. These will be the first major public tests of the police's efforts.
Looking more broadly, the government says there are further measures it has taken to protect society, one of these being the current Civil Contingency Bill which is designed to beef up how we protect society in times of national emergency.
But the view from elsewhere is completely different.
Human rights campaigners, pressure groups and increasingly members of Britain's Muslim communities oppose the anti-terror laws because of the way they say they are being abused.
They believe that some police officers are not using the laws properly and are picking up people because of the way they look, rather than on hard evidence of a credible threat.
For instance, a major source of concern outside of terror laws themselves, is the use of stop and search powers.
There has been a 300% increase in stops of people classed as "Asian", although as an absolute figure it comes to about 3,000 people.
There are a string of on-going campaigns against terror laws. One of the largest, Stop Police Terror, sees its meetings attended by hundreds of people.
It was launched in the wake of the arrest of a south London man, Babar Ahmad, who was recently re-arrested and faces extradition to the United States.
But do they have hard evidence to support their criticisms of unfair targeting?
Many of fears are based on anecdotal reports passed on through communities. But the Institute of Race Relations, a think tank, has just completed its own survey of anti-terror arrests.
It says the majority of those being arrested are Muslim - but the majority being convicted are not. This, it claims, means the wrong people are being targeted.
How does the government respond to this kind of criticism?
The Home Secretary David Blunkett says he is very clear about the use of terrorism laws. They are there to target terrorists, rather than individual members of one community or another.
He has also recently announced plans for a new law to target incitement to religious hatred, something he believes will help protect the rights of Muslims, and all other faith groups, from bigotry in a time of heightened tensions.
Police chiefs have also written detailed defences of anti-terrorism legislation aimed at British Muslims, or have spoken in person at public events.
Is this reassurance enough?
The plans to tackle religious hatred have been welcomed by many Muslims - but there is considerable concern that their community is being tarnished with the brush of terrorism because of a combination of sloppy police work, general fear over immigration and media stereotyping.
The Muslim Council of Britain is about to launch a new campaign of "Know Your Rights and Responsibilities" to help ordinary people feel protected and prepared.