Portuguese author and journalist Miguel Sousa Tavares is under investigation for calling President Anibal Cavaco Silva "a clown" in an interview.
The famous writer said his country did not need someone like Italy's comedian-turned-politician Beppe Grillo, because it already had a clown as leader.
He faces up to three years in prison if found guilty of offending the honour of the President of the Republic.
Mr Cavaco Silva has lost public support over his cabinet's austerity policies.
Recent polls show he has the lowest ratings of any Portuguese president ever, the BBC's Alison Roberts, in Lisbon, reports.
By contrast, Mr Sousa Tavares's work is hugely popular, both in book form and adapted for TV, our correspondent says.
He made the controversial remark in an interview with Portugal's leading financial daily, Jornal de Negocios.
"We already have a clown: His name is Cavaco Silva," he told the newspaper.
The quote was also splashed in large letters across the newspaper's front page.
The author later said he went a bit too far in his comments.
Meanwhile, the attorney-general's office has not specified if the move to investigate the novelist was prompted by the president himself.
If criminal proceedings were to run their full course, it might not do much good for Mr Cavaco Silva's image, our correspondent says.
Instead of being seen as above party politics, the president has become associated with the centre-right government and its austerity policies, she adds.
The measures have triggered mass protests across the country.
This year's budget was Portugal's toughest in living memory, imposing tax rises that for many workers amount to a month's wages.