The renowned Royal Shakespeare performer complained of being unwell 10 minutes from the end of a production of The Man Jesus and had to be helped off the stage.
The incident happened during a preview performance of the play which is due to open officially on Wednesday.
The one man show sees Callow depict the life of Jesus Christ through the eyes of those around him. It is due to run at the Lyric Theatre until 20 April.
During Tuesday night's performance members of the audience said Callow at one point appealed for help after becoming faint.
Someone from the audience then got onto the stage to help the actor, while another person came from the back of the theatre.
Meanwhile a press call for the play in which Callow was due to attend on Wednesday afternoon has been postponed until further notice.
However it has been announced the play will be performed on Wednesday evening as scheduled.
Richard Croxford, Artistic Director of the Lyric Theatre said: "The show will go on. Simon Callow is feeling much better and we are delighted to announce that the world premiere of The Man Jesus will open tonight and run until 20 April."
Callow is a well-known stage actor who has performed iconic roles such as Oscar Wilde in The Importance of Being Earnest.
He is also a star of the screen having appeared in films, including Four Weddings and a Funeral and Phantom of the Opera.