Agyeman was relatively unknown when she was picked for the role

Tickets for the special screenings at Cardiff and Swansea, which have been used as locations, as well as Aberystwyth, Wrexham and Pwllheli, were snapped up within hours.

The rest of the nation have to wait until 1900 BST when the opening episode of the series will be shown.

Swansea-born writer Russell T Davies has said a fourth series is planned.

The first episode of the third series, introduces viewers to Doctor Who's new assistant, a trainee doctor called Martha Jones, played by Freema Agyeman.

She replaces former assistant Rose Tyler who was written out at the end of the second series after Billie Piper decided to leave the award-winning show, which is produced in Wales.

Tennant has refused to answer speculation about when he will quit

Agyeman appeared briefly in an earlier episode, playing a minor character who suffered at the hands of the Cybermen.

Although most details of what will happen in the new series have been kept tightly under wraps, it is known that the time lord, played by David Tennant, shares a kiss with his new co-star in the opening episode.

Speaking at the London premiere of the new series earlier this month, Tennant was keen to praise the Welsh capital where much of the series is filmed.

"Cardiff is a great city and has been very good to us," he said. "It's lovely to film there because everyone is so enthusiastic.

"You try filming in London and everyone's so mean about it and complains that there is a huge camera truck parking in their street. Everyone in Cardiff seems to welcome us with open arms."

Also at the premiere, executive producer and writer Davies confirmed Doctor Who would return for a fourth series in 2008.

"Series four is officially existing. I'm very excited, but we have known for ages," he said.

The 1,200 tickets for screenings at Odeon cinemas in Cardiff, Swansea and Wrexham, as well as at Aberystwyth arts centre and Neuadd Dwyfor, Pwllheli, went within hours.