The arrest of shadow minister Damian Green this week continues to spark debate.
Mr Green was held and questioned for nine hours on Thursday "on suspicion of conspiring to commit misconduct in a public office and aiding and abetting, counselling or procuring misconduct in a public office".
His homes and offices, including in the House of Commons, were searched and his computer, phone and other communications equipment seized in the raids.
Mr Green was not charged with any offence after his arrest on Thursday but was released on bail until February, when he could face further questioning.
JACQUI SMITH, HOME SECRETARY
There have been a lot of charges thrown around here - the idea that, you know, this is Stalinism, this is a police state.
In my book, Stalinism and a police state happens when ministers direct and interfere with specific investigations that the police are carrying out.
And I have been very clear, that in my view, the police should have operational independence - they should be able to pursue investigations in the way in which their professional judgement suggests.
If you believe in the operational independence of policing. If you believe that the professional judgement of police officers, during the course of an investigation, should be able to take its course, well you believe that even when things are tricky and sensitive.
It started as an investigation of a systematic series of leaks from a department that deals with some of the most sensitive, confidential information in government. A systematic series of breaches of security effectively - and the idea that my permanent secretary or the cabinet secretary would not have been concerned about this, I think, is frankly unbelievable.
DAVID CAMERON, OPPOSITION LEADER
David Cameron is unhappy with the prime minister's stance on the arrest
If this approach had been in place in the 1990s, then Gordon Brown would have spent most of his time under arrest.
The Prime Minister has simply repeated that he 'had no prior knowledge' and this is 'a police matter'. Frankly, that's not good enough.
Does Gordon Brown think it is right for an MP who has apparently done nothing to breach our national security - and everything to inform the public of information they're entitled to know - to have his home and office searched by a dozen counter-terrorist police officers, his phone, blackberry and computers confiscated, and to be arrested and held for nine hours?
Of course no one is above the law. But in a democracy there is an important line to be drawn when it comes to acting in the public interest.
KEN CLARKE, CONSERVATIVE MP
Outrageous... There is no crime, this is an abuse of police powers, this is President Nixon's America - harassing a political opponent of the government. It should stop.
HARRIET HARMAN, LABOUR MP AND HOUSE OF COMMONS LEADER
We have got to be sure that whilst MPs are not above the law, that actually they are able to get on with their job without unwarranted interference by the law.
These are very, very big constitutional principles, we have to make sure they are protected.
NICK CLEGG, LIBERAL DEMOCRATS LEADER
This is breaking with centuries of tradition about the independence of Parliament, the confidentiality of information kept by MPs.
And to do that on the nod, because the police say they want to, without telling your political masters? It's either implausible or its extraordinarily incompetent.
SHADOW FOREIGN SECRETARY
I don't think it's possible for the government just to say, 'Oh we're only the government, we don't really have any interest in this and we don't know the answers to any questions'.
They are going to have to answer some questions, and they are going to have to answer them pretty soon.
SHADOW HOME SECRETARY
She [Jacqui Smith] is the person who is ultimately accountable to Parliament for police failings.
If it turns out, which I think it may well do, that the police were barking up the wrong tree and pursuing an MP doing his legitimate job, then I think she has a great deal to answer for.
All sense of proportion and common sense seem to have been lost. I don't think this would have happened in the past.
Home Secretaries would not have intervened operationally but they would have said 'hang on a minute, are you sure you know where you are going, have you consulted with the law officers, are you really sure about the allegations?'
All these checks, which I would expect to see work in a healthy democracy, seem to have been abandoned and I think we should be worried about that.
MICHAEL HOWARD MP,
FORMER CONSERVATIVE LEADER
I don't know what Jacqui Smith knew or what Gordon Brown knew. I do know that Gordon Brown was the man who made use of leaked information time after time after time when he was in opposition.
It's quite outrageous and Gordon Brown should say so. For Gordon Brown to remain silent on the matter when he knows that he behaved in exactly the same way as Damian Green has behaved is absolutely indefensible.
I believe there should be an apology to Damian Green for what happened.
Of course you want to stop the leaks but I never suggested that action should be taken against the people receiving the leaks.
It's one thing to want to stop the leaks and take action against the civil servants who are leaking, if that's the case, but it's a million miles to say: 'Let's arrest the opposition members of parliament who are benefiting from these leaks'.