An Oslo police spokesman said: "We are watching closely what is being posted on the internet. Anything we believe would threaten his personal safety will be taken seriously."
Former international referee Graham Poll said today: "This morning he's being smuggled out of our country under police escort — this is a referee of a football match. That is a disgrace.
"When he booked in a hotel they had to change the hotel he was staying at because of the fear that maybe fans would find him."
Poll blamed the players for stoking the fans' anger. He added: "When you see players acting like that, fans are called fans because they are fanatics — they may feel it's time to take action themselves."
He said that Drogba and Ballack's behaviour was "a dreadful display of petulance", adding that stewards at Chelsea had failed to step in quickly enough to protect the referee. "Players should show some restraint. They should show some respect."
The controversy began when Ovrebo denied Chelsea a series of penalties in last night's match.
The referee was branded a "f***ing disgrace" by Chelsea striker Didier Drogba ,and was today accused of taking a bribe by defender Jose Bosingwa, after denying them four penalties.
The campaign against Ovrebo has echoes of the case of Anders Frisk, a referee who faced a hate campaign after another Chelsea versus Barcelona match. Chelsea's coach at the time, Jose Mourinho, accused him of "adulterating" the outcome of a match. Frisk eventually quit as a referee.
On Yahoo Answers, a user has posted Ovrebo's address in Oslo. Threats were made on Chelsea fans' forums including CFCnet and The Shed End. One fan wrote: "I want that ref assaulted" while another said: "I will kill him."
On Facebook, several groups were created by this morning dedicated to abusing the referee, including one titled "Kill Tom Henning Ovrebo".
The creator of one group states: "We group dedicated to the hunt for (and brutal murdering of) the second-leg champions league ref of Chelsea vs Barcelona." Mr Ovrebo was pursued into the players' tunnel by Drogba and Ballack with the Ivorian hitting the wall with his fist as abuse was screamed at the official.
The witness said one player shouted: "Tell the world that Uefa didn't want Chelsea in the final." There have been suggestions that Uefa does not want a second all-England final after last year's game between Chelse and Man Utd.
Ovrebo, 42, works as a psychologist outside football. In Euro 2008, the Italian Football Federation demanded an apology from Uefa after he incorrectly ruled out a goal by Luca Toni in Italy's match against Romania. Afterwards, Ovrebo admitted he had made a mistake and was not assigned any more tournament matches.
