LONDON (REUTERS).- Portraits of three different lovers of 20th century master Pablo Picasso fetched the three highest prices at a London auction at Christie's on Tuesday, the first in a key series of art sales over the coming weeks.

Top lot on the night at the impressionist and modern art evening sale was a depiction of Dora Maar, who became Picasso's lover and muse at the expense of Marie-Therese Walter.

The 1939 work, which had been unseen in public since 1967, sold for 18.0 million pounds ($29.1 million), several times the pre-sale estimate of 4.0-8.0 million pounds.

The second highest price paid on the night was for "Jeune fille endormie," a 1935 portrait of Walter that went under the hammer for 13.5 million pounds ($21.9 million).

The painting, valued at 9.0-12.0 million pounds, was given to the University of Sydney in 2010 by an anonymous donor on the condition that it was sold and that the proceeds went to the university to fund scientific research.

"When they gave us this remarkable work our donor said 'this painting is going to change the lives of many people'," said Michael Spence, Vice-Chancellor of the University of Sydney.

"They were right. We are grateful for their extraordinary generosity and delighted with the outcome of the auction."

In third place was "Buste de Francoise" (1946), which fetched 10.7 million pounds. Francoise Gilot was an artist and author who became Picasso's lover in the 1940s and with whom he had two of his children, Christie's said.

Overall the auction raised 140 million pounds, according to preliminary, unofficial results on the auctioneer's website, versus expectations of between 115-165 million pounds.

Christie's rivals Sotheby's holds its equivalent sale in London on Wednesday.

(Reporting by Mike Collett-White)