Rewards totalling £2.5m have been offered to anyone who can help with information leading to the safe return of Madeleine McCann.
Poster campaigns have begun in Portugal and the UK
The News of the World and businessmen including Sir Richard Branson have jointly pledged £1.5m. Scottish tycoon Stephen Winyard has offered £1m.
Madeleine's fourth birthday has been marked in the UK and in Portugal, where she disappeared on 3 May.
Posters have been distributed and many have joined an e-mail campaign.
Madeleine, from Rothley, Leicestershire, disappeared from an apartment in the resort of Praia da Luz.
Fifty pink balloons were released at the village of Queniborough, close to Rothley, on Saturday, where Madeleine lived until a year ago.
Yellow ribbons were tied to railings in the family's home village, while others were worn by Celtic and Aberdeen fans at a football match in Glasgow. Posters were handed out to holidaymakers leaving from East Midlands Airport.
Mr Winyard said of his decision to offer a reward that he would be "rejoicing with the rest of the nation" if it helped to secure Madeleine's safe return.
The News of the World has promised £250,000 - matching the contribution of Top Shop owner Sir Philip Green.
Sir Richard and Everton FC owner Bill Kenwright were among others who contributed.
Madeleine's parents, Gerry and Kate McCann, who are still in Portugal, released a statement on Saturday pleading for people to "redouble their efforts" to find Madeleine.
"Keep looking, please keep praying, please help bring Madeleine home," they said.
The girl's aunt, Philomena McCann, said the couple would be having a "family day" to mark Madeleine's birthday, and celebrate her life so far.
Other family members gathered at the Glasgow home of Madeleine's uncle, John McCann, she said.
British ex-pats met on Saturday lunchtime at the holiday complex where Madeleine was taken.
They tied yellow ribbons on the railings to express their hope that she would be returned safely.
In the UK, Chancellor Gordon Brown was among those expressing sympathy for the family, saying every parent would be thinking of ways to help them.
Several football players in Scotland, where Madeleine's father is from, made appeals for information. This follows televised messages in recent days from stars including John Terry, Cristiano Ronaldo and David Beckham.
The official ground search for Madeleine in the Algarve has ended, but police say they still have significant leads to follow up.
BBC correspondents say detectives have questioned nine people in the past few days, but say there is little sign of a breakthrough.
The focus is shifting from being a local search for Madeleine to an international child abduction inquiry, amid suggestions she may have been taken out of Portugal.
Media reports about leads have focused on CCTV footage from a petrol station on the night of Madeleine's disappearance.
It was thought to feature two men and a woman driving a car with a British number plate.
Police have declined to confirm or deny reports about any possible leads, citing Portuguese law.
The McCann family have been actively campaigning for publicity for the case, hoping poster and e-mail campaigns will help the police.
The international number for Crimestoppers is +44 1883 731 336. People with information about Madeleine can call anonymously.