The 83-year-old, who suffers from Parkinson’s disease and can no longer walk, appeared markedly better at his general audience in St Peter’s square on Wednesday, a day before the anniversary.

He spoke in a voice that was much clearer than it has been recently and clearly recognised cardinals who greeted him at the end of the audience for thousands of people.

He invited everyone to join him in St Peter’s square on Thursday evening for a mass at about the same time as he first appeared to the world as Pope on 16 October 1978, minutes after he was elected the first non-Italian pontiff in 455 years.

Among the cardinals was Bernard Law, the former Archbishop of Boston who resigned after a child sex scandal that began in his diocese and swept across the United States last year.

Nuns, children, workers and even US astronauts are descending on the Vatican for the occasion.

“It is very exciting, almost as exciting as being in space, maybe more exciting,” said Mike Massimino, who took part in a 2002 mission abroad the space shuttle Columbia.