Norway's Wig Wam are among the expected frontrunners
Ten semi-finalists will go through to join performers from 15 other countries who have automatically won slots, including the United Kingdom's Javine.
The contest is being staged in Kiev, capital of Ukraine, last year's winner.
The semi-final will be broadcast live on BBC Three from 2000 BST. The final will be broadcast simultaneously on Saturday on BBC One and Radio 2.
Bookmakers in London put Greece's Helena and the Norwegian band Wig Wam as the odds-on favourites in the final.
France, Germany and Spain are among the other countries already in the final.
In the semi-finals, Switzerland's all-female Vanilla Ninja, Hungary's Nox and Bosnia-Herzegovina's Feminem are among the favourites to make the cut for the finals.
Eurovision is one of Europe's major TV events, with more than 150 million people expected to watch the final.
The winner is determined by telephone voting in each country. This year's event is the 50th to be staged.
The most famous Eurovision winners of all-time are Sweden's ABBA, who followed their 1974 win with Waterloo with a string of worldwide hits.
Eurovision contestants are characterised by the catchy melody of their songs, vivid outfits and energetic stage shows. Many draw heavily on their countries' musical traditions.
Last year's winner, Ruslana, combined both elements by drawing on the music of Ukraine's wild Carpathian Mountains region while wearing leather and fur and doing a modern dance routine.
Ukraine is unlikely to notch up a second consecutive win. Bookmakers have see its entry Green Jolly as an 80-1 shot.
One of their rap songs was the unofficial anthem of last year's Orange Revolution protests.Huge crowds sang their songs when they gathered in downtown Kiev to protest against the presidential election results.