Prime Minister's Questions - very nasty.

Poor Lindsay Roy. It was the Glenrothes headmaster's first day in the Commons and the classroom was on its worst behaviour.

Prime Minister's Questions is a barracking, gladiatorial confrontation each Wednesday, but there were seasoned commentators yesterday who could not remember such vitriolic exchanges as took place while dealing with a harrowing and tragic subject.

The spark that set off the explosion was a question from Tory leader David Cameron on the case of Baby P, beaten and left to die by his guardians despite being under the care of Haringey Council in north London.

Gordon Brown has underestimated David Cameron in the past. Yesterday he made the error of mis-reading the man. The Prime Minister was prepared, as he has been in the past few weeks, to wipe the floor with the opposition leader over the economy.

The stage was set for the battle - jobless total rising, Bank of England predicting dire forecasts of the economy. There were even a few helpful early economics questions lobbed in by friendly backbenchers and unwitting opposition MPs which had the government benches roaring.

Mr Cameron rose to his feet, saying only the Prime Minister could look so "smug on the day that 140,000 lost their jobs". But then he changed the script, reading off the front page news about the Baby P case.

The PM gave a stock response about the nation being shocked and saddened by what had happened. There was a national review of practice, said Mr Brown, and a report just arrived from Haringey on which the government would take action.

Mr Cameron demanded more than a stock response. Why, he asked, was the person who runs the children's service department in the council having a role in writing that report and investigating her own actions?

Labour MPs suspected he was trying to avoid asking the obvious about the economy and bayed like wolves. Mr Speaker called for order.

The Tory leader was angered by their attitude, his papers went flying as he became more passionate on the subject. Mr Brown gave the stock answer again, groping to find some cross-party unity, but unable to help himself from adding: "I do regret making a party political issue of this."

That infuriated Mr Cameron. "What the Prime Minister has said just now is frankly cheap," he said. "I ask the Prime Minister to withdraw the remark that it was about party politcs."

Twice he asked Mr Brown to withdraw the allegation, and twice Mr Brown blustered on with his slim briefing on the subject. The Commons was in uproar. The Speaker had to appeal to the MPs not to shout across the chamber while dealing with such a horrifying subject. They carried on shouting in an unedifying performance that left everyone discomfited.

A third time, a furious Mr Cameron asked the PM to withdraw the accusation. He would not let go, drawing in Westminster Children's Minister Ed Balls as someone who had also highlighted the problem of social service directors investigating their own conduct.

Mr Brown probably thought that Mr Cameron's finger-jabbing, desk-banging anger was a construct. It didn't look like that from the press benches and it certainly won't have looked like that on TV last night. Contrast, said Labour sources afterwards, Mr Cameron's tone with that adopted by Lynne Featherstone, the LibDem MP who had been leader of the opposition in Haringery Council during the Victoria Climbie scandal. Later she called, calmly, for an independent inquiry into something that was never meant to happen again.

The opposition leader might have ducked another drubbing on the economy but, in the exchange, Mr Brown was exposed as the man he was before the economic storm lifted him from the doldrums. He looked a lumbering, disconnected leader, incapable of nimble footwork, unable to see a Tory, on any issue, as anything other than an enemy to be flattened.