Pope John Paul II, suffering from 'flu, yesterday cancelled all his appearances during his trip to Poland. It is unclear what effect his sudden illness will have on the remainder of his visit.
It was the first time during 87 international visits that the Pontiff (79) has had to cancel events because of illness.
He was forced to call off appearances in his beloved Krakow, the southern city where he was cardinal until his election as Pope in 1978.
More than a million people had gathered in the rain to see him celebrate Mass. A collective gasp went up and people began crying when a priest announced that the Pope was ill and that the Mass would be dedicated to prayers for his health.
Vatican spokesman, Mgr Joaquin Navarro-Valls, a doctor, said since Monday evening the Pope had been suffering from "a moderate fever of a probable viral nature".
He said the Pope would rest for the day in the Krakow archbishop's residence, his home until 1978. "Besides rest, the Pope will be undergoing an anti-influenza therapy, the usual thing in these cases."
He said the Pope's fever had remained under 38Centigrade (100.4F) and there was "absolutely no connection" with a fall last Saturday for which he needed three stitches in his forehead.
Mgr Navarro-Valls said the Pope was already feeling ill on Monday afternoon but he went ahead with an event in the mining city of Sosnowiec.
No decisions have been made about the remainder of the Pope's 13-day trip to his homeland. It is due to end on Friday when he is scheduled to make a short visit to Armenia to visit the country's patriarch, who is suffering from cancer.
The Mass at Krakow's huge Blonie Park was celebrated on the Pope's behalf by Cardinal Angelo Sodano, his secretary of state and top aide. The homily the Pope had prepared was read to the crowd by Cardinal Franciszek Macharski, his successor as Archbishop of Krakow.
The Pope, believed to be suffering from Parkinson's Disease, had planned visits to 21 towns and cities during his Polish trip.
People in the Krakow crowd were devastated when the announcement was made. "This is a catastrophe. I can't believe it. I will pray for him," said Marcin Borcz (14), an altar boy.
The Pope has suffered fevers in the past. The last time he was known to be suffering from influenza was two years ago when he had to cancel several activities at the Vatican.
He has had health problems since 1992, when he had major surgery to remove a colon tumour the size of an orange which was beginning to turn malignant. The Vatican has repeatedly denied reports that the cancer had spread.