Opposition Leader Tony Abbott says he regrets some protesters at yesterday's anti-carbon tax rally in Canberra used abusive language.

Protesters at the rally outside Parliament House held placards with slogans like "Ditch the witch", while one banner labelled Prime Minister Julia Gillard "Bob Brown's bitch".

Senator Brown wrote to Ms Gillard saying he was appalled Mr Abbott spoke in front of such offensive banners.

Climate Change Minister Greg Combet says it was not appropriate for Mr Abbott to address the rally while standing in front of some of the signs.

"He should apologise to the Prime Minister for being associated with such despicable placards," he said.

In a statement, Mr Abbott says he regrets abusive and hurtful language was used by a small number of the protesters and he does not endorse it.

Mr Abbott told the crowd of about 3,000 people they did not look like environmental vandals or scientific heretics.

He said he did believe humans were contributing to climate change but that the current debate was a matter of political honesty.

"There are a lot of diverse opinions about climate change," he said.

"Climate change happens. Mankind does make a contribution. It is important to have an intelligent response, not a stupid one."

Opposition climate change spokesman Greg Hunt has accused the Government of double standards.

"The ALP website right now is comparing Tony Abbott with a Nazi war criminal," he said.

The Opposition earlier this week accused Ms Gillard of making veiled comparisons to the holocaust by repeatedly referring to Mr Abbott as a climate change denier.

But the Prime Minister maintains there is nothing untoward about her use of the word.