IF you'd switched between Channels 4 and 5 around 11 o'clock last Thursday, you'd have seen Martin Sheen on both. And by coincidence, he was playing the President of the United States in both.

Not the same President and not a real one either. His regular role in the White House-set series The West Wing and his one-off appearance in the TV movie Medusa's Child are fictional characters.

But if a B-movie actor can get to play the President for real - remember Ronald Reagan? - then it's only fair that ordinary working actors should have the chance to play at being the most powerful man in the world.

American leaders are a constant source of drama for film-makers with two new movies adding both real and fake versions of the man in the White House. Canadian actor Bruce Greenwood becomes the latest actor to impersonate John Kennedy in new cinema release Thirteen Days, a recreation of the 1962 Cuban Missiles Crisis. Then next month, Jeff Bridges appears in the political thriller The Contender as President Jackson Evans, whose decision to appoint a woman as his Vice-President causes a furore.

Here's a top ten of the Presidents - some existed, others are figments of a scriptwriter's imagination - that have seized power on screen.

President James Marshall

(Harrison Ford in Air Force One).

He proves not just a man of words but a man of action too when the Presidential plane is hijacked by Gary Oldman's foreign terrorist gang. The First Lady looks on in amazement as the President turns into Han Solo and Indiana Jones rolled into one and kicks some terrorist butt in mid-air.

President John Kennedy

(Cliff Robertson in PT 109).

THE adventures of a young, pre-Presidential JFK in the navy, in the Pacific, during the Second World War. The Kennedy family has featured in a number of films and TV mini-series but not, we are supposed to believe, in The Greek Tycoon. This movie, about the widow of an assassinated US President who marries a billionaire shipping magnate, claimed that any resemblance to real people was coincidental.

President Richard Nixon

(Anthony Hopkins in Nixon).

BRIT Sir Anthony (Hannibal Lecter) Hopkins was an unlikely choice to impersonate the disgraced US President but he turned in a creditable performance in director Oliver Stone's biopic about events surrounding Watergate. Mrs Nixon was played by Joan Allen - who now plays the hopeful Vice President in The Contender.

President James Dale

(Jack Nicholson in Mars Attacks!).

WHEN raygun-toting Martians arrive with plans to take over Earth and annihilate the human race, Jack Nicholson's President leads the peace negotiations with all the tact of Anne Robinson talking about the Welsh.

President Bill Mitchell

(Kevin Kline in Dave).

When the unpleasant President is left in a coma after a stroke, a lookalike (Kevin Kline again) is recruited to stand in for him on official duties, in this political comedy. The trouble is everyone, including First Lady Sigourney Weaver, prefers the nicer substitute to the real one.

President Whitmore

(Bill Pullman in Independence Day).

Another bunch of aliens arrive aboard dozens of spaceships and take up positions to wipe out the world's major cities. Pullman's former fighter pilot leader takes charge of zapping those spacecraft out of the sky.

President Tom Beck

(Morgan Freeman in Deep Impact).

Another President, another global catastrophe. This time not visitors from another planet but a comet hurtling towards the Earth on a collision course that would wipe out life as we know it, Freeman's Tom Beck organises a lottery for places in the underground shelters. Fortunately, Dale Winton doesn't introduce the TV coverage.

President Woodrow Wilson

(Alexander Knox in Wilson).

Expensive bio-pic charting the story of Wilson from head of Princeton University through the governorship of New York to US President during the First World War. Won several Oscars and a best actor Golden Globe for Knox

President Abe Lincoln

(Henry Fonda in Young Mr Lincoln).

Hollywood bio-pic following Abe's early days from country boy to lawyer, directed by John Ford.

Would-be President Jack Stanton (John Travolta in Primary Colours).

Political satire adapted from a book influenced by Bill Clinton's campaign. Travolta's Southern governor tries to dodge scandal and double-dealing on his way to the White House. Emma Thompson is his wannabe First Lady. Not to be confused with Wag The Dog in which a Presidential aide and Hollywood producer invent a foreign war to divert attention from a President caught up in a sex scandal involving a Girl Scout.