Soccer-Porto sell Martinez to Atletico, buy Imbula from Marseille

LONDON, July 1 (Reuters) - Porto agreed to sell their Colombia striker Jackson Martinez to Atletico Madrid while agreeing a deal to sign France Under-21 international Giannelli Imbula from Olympique Marseille on the opening day of the summer transfer window on Wednesday. Imbula signed for 20 million euros ($22.3 million) from Marseille, with Porto revealing he has a release clause of 50 million euros. The Belgium-born 22-year-old plays as a defensive midfielder and is tipped for a fine future. Atletico moved to strengthen their forward line for next season following the exit of Mario Mandzukic by agreeing to buy Martinez. Martinez, 28, had a contract with the Portuguese club until the end of the 2016-17 season and Atletico agreed to pay his buyout clause worth 35 million euros, Porto said on their website (www.fcporto.pt). Diego Simeone's side, who are yet to confirm the deal, were in the market for a striker after they sold Croatia international Mandzukic to Italian champions Juventus. Simeone is looking to rebuild after Atletico failed to reach the heights of 2013-14, when they won their first La Liga title in 18 years and came close to beating Real Madrid in the Champions League final. Martinez graduated from the Independiente Medellin youth academy to the first team before joining Mexico's Chiapas Jaguares in January 2010 and then moving to Porto in 2012. ($1 = 0.8970 euros) (Reporting by Mike Collett and Iain Rodgers; editing by Justin Palmer)