A man is in a serious condition in hospital after setting himself alight outside a Brisbane courthouse today.

The 60-year-old man set himself on fire in the forecourt of the Brisbane Supreme and District Court building at about 2pm.

It is understood security staff rushed to the man's aid, before paramedics arrived.

He suffered burns, ranging from first-degree to third-degree, to more than 75 per cent of his body and was rushed to the Royal Brisbane Hospital in a critical condition.