Automated phone calls in which actor Sir Sean Connery urges thousands of Scottish households to vote for the SNP have been defended by the party.
Sir Sean Connery is a long-time independence supporter
The Liberal Democrats claimed the Nationalists could be fined for contacting people without consent.
But the SNP hit back with leader Alex Salmond insisting legal advice supported the automated calls.
Thousands of households would have received messages from the actor and SNP supporter earlier this week.
Meanwhile, the Tories claimed they are better placed than Nationalists to defend Scottish interests like fishing.
Chancellor Gordon Brown endorsed Labour's policies and the Green Party launched its election campaign.
The UK Information Commissioner, Richard Thomas, has written to the SNP saying the party would be breaking telephone marketing rules if the calls are made without the consent of those receiving them.
Mr Thomas' move was disclosed in a letter to the Liberal Democrats, who raised the issue with the commission.
He told Lib Dem MSP Iain Smith: "Although there is nothing to say that the SNP has not got the consent of the people it intends to contact, I have today written to remind the party that without consent such calls would contravene the regulations."
Mr Smith, the Liberal Democrats' campaign chairman, said: "I think it very unlikely that the SNP has the prior permission of the 500,000 Scots they claim were to receive the pre-recorded message.
"The Information Commissioner has confirmed our belief that such calls, without prior consent, are illegal."
The 35-second pre-recorded message from Connery, a long-time SNP supporter, begins: "Hello there. This is Sean Connery.
"No, it's not a joke - unfortunately the real joke is the Labour party."
The call goes on to proclaim the SNP as "the most trustworthy political party of them all" and urges people to vote for them.
The commission issued guidance to political parties when the election was called earlier this week.
The guidance says: "It is worth noting that many individuals tell us that they consider such calls to be extremely intrusive and even disturbing."
A spokeswoman for the commission declined to comment on the Connery calls, saying she did not know the content of the calls or whether the SNP had consent.
SNP leader Alex Salmond branded the claim by the Liberal Democrats as "inaccurate".
He told BBC Scotland's news website: "We have legal advice that strongly supports our position. They have nothing more than supposition and allegation.
"Quite apart from the detail of the way we are conducting this campaign, we are a political party engaged in an election not a commercial marketing organisation.
"At the most basic level, we are protected under Article 10 of the European Convention on Human rights, on freedom of speech."