Hugh Grant has accused the Metropolitan police of behaving in a "worrying and deeply mysterious" way after Scotland Yard invoked the Official Secrets Act to demand journalists reveal their sources.

As a senior Liberal Democrat called on the attorney general to block the "extremely bizarre" use of the act, Grant warned that police were turning on the "goodies" after Scotland Yard applied for an order under the 1989 act to require the Guardian to identify its sources on phone hacking.

Speaking at the Lib Dem conference in Birmingham, the actor said: "It is a very worrying and upsetting development. A lot of us victims and campaigners had come to the view that the new police inquiry – [Operation] Weeting under Sue Akers – were good cops.

"It was a new investigation. They were embarrassed by the behaviour of their predecessors and colleagues. So for them to suddenly turn on their fellow goodies in this battle is a worrying and deeply mysterious."

Grant spoke up as Don Foster, the veteran Lib Dem MP, said that the attorney general, Dominic Grieve, should use his discretion to rule that the use of the Official Secrets Act in this case was not in the public interest.

"I understand the attorney general has the opportunity to use this power," Foster said after a fringe meeting, organised by the Hacked Off campaign, that was addressed by Grant. "He should use it and say this is not in the public interest."

Foster, who praised the Guardian for "fantastic journalism" in exposing phone hacking, found unanimous support at the fringe meeting when he asked whether the Guardian's disclosure that Milly Dowler's phone had been hacked – the revelation that prompted the police use of the Official Secrets Act – was justified.

The MP said: "If it was in the public interest for the Guardian to do what they did it is extremely bizarre, it is almost unheard of, for the Metropolitan police to have used the Official Secrets Act as the basis for seeking to get hold of the information they want. Bizarre because what it does is it means that what you and I have just demonstrated by our vote – that the issue of public interest will not be able to be judged.

"That is why it is absolutely vital that we find out first of all who actually signed off the agreement to use the Official Secrets Act and, secondly, we have to have a very, very clear explanation of why they are doing it. A final decision is made by the attorney general as to whether to allow it to happen.

"The one good bit of news is that, in making his decision, the attorney general can use public interest as one of the criteria that he considers."

Tom Brake, chair of the Lib Dem home affairs committee, echoed Foster's comments. "The Guardian is to be commended for the work that it has done to bring the facts of the phone hacking scandal to light. The revelation that Milly Dowler's phone had been hacked was the trigger for a political earthquake that went on to engulf the press, politicians and the police.

"The use of the Official Secrets Act in these circumstances is very unusual, and all the more worrying because it does not allow the defendant to argue that their actions were in the public interest. The Met need to explain why they think it is appropriate to use the Official Secrets Act in this case.

"While this is clearly a matter for the police and the attorney general, I do question whether this action is in the public interest given everything that has happened, or indeed in the interests of investigative journalism."

The Lib Dem anger over the use of the Official Secrets Act came as Hugh Grant met Nick Clegg for what he described as a "delicious" seven-minute lunch in Birmingham at the start of a tour of the three party conferences to ensure that politicians continue to show "balls" on hacking after the establishment of the Leveson enquiry.

Grant said he was disappointed after the deputy prime minister said it would take time for legislation to be introduced after the Leveson inquiry reports.

"Nick Clegg did slightly depress me by saying that nothing could be done in terms of legislating until Leveson reports. So that is a snag. But I comfort myself with the fact that Leveson, as it progresses, will be reported. I hope this will keep the scandal in the forefront of the news.

Grant added: "Ultimately it is going to be politicians who get the job done, who get the thing fixed. So I am here with Hacked Off to have a look at the politicians in all three parties and see which of those politicians who appear to have grown balls in July actually still have them and get something done."

"The judgment is yet to be made. They had no choice back in July. The revelations were so shocking to the whole country that they had to talk a good game. Whether or not they will now play a good game really remains to be seen. That is one of the reasons we are going to these conferences – to put pressure on them to make sure they do as they said they'd do."

Grant said Britain was shamed by two newspaper industries in Britain. One is the cornerstone of Britain while a second, which used to be interested in journalism, uses criminal means to appropriate the privileges of British citizens.

"I am keen for people to stop conflating these two industries. I am keen for them to stop saying, 'we can't throw out the baby with the bathwater.'

"Well to me and most sane people it is very easy to distinguish baby and bath water. It is very easy. You take the baby out of the bath and, in fact, I would argue the baby is now quite big enough to get out of the bath itself and it is high time that good journalists, broadsheet journalists, just get out of the fucking bath."

Grant was also critical of the House of Commons culture select committee for its cross-examination of Rupert and James Murdoch.

"Speaking as a bad actor myself I thought Murdoch's performance was dodgy. Many of my sources tell me he was a hell of a lot sharper than that a week before – people who had met him. I didn't buy the long pauses. Much as I adore the people on that committee, like Tom Watson … I was shocked at how unsharp and how slightly starstruck the select committee members seemed.

"I bitterly regret the cream pie incident because it absolutely played into his hands. I still don't know why they had to congratulate him so much afterwards for being so brave or why my hero Tom Watson had to say your wife has a great right hook.

"I mean for God's sake this was the one chance we have ever had to get the guy in the dock and suddenly everyone was slightly up his arse."

Grant, who famously posed as a journalist from Horse and Hounds in the film Notting Hill to interview Julia Roberts, said that granting an interview does not give the media a right to pursue a celebrity.

"The papers don't give people privacy for free. It is done as a sort of barter when it is done. If I give an interview to a magazine they get something out of it, I get something out of it. But the deal is over. If I have sold you a pint of milk for 50p you can't come to me forever after saying you once sold milk, I can help myself to your milk for free. It is patently absurd."

There was a lighter moment Grant was asked how he would play David Cameron. "I only ever play one part. Don't be ridiculous."