BUENOS AIRES, July 5 (Reuters) - Argentina's business-friendly PRO party appeared to win Sunday's mayoral election in capital city Buenos Aires, a key electoral district, but not by a big enough margin to avoid a run-off vote later this month, early official returns showed.

With 59 percent of the vote counted Horacio Rodríguez Larreta, chief of staff to outgoing mayor and presidential candidate Mauricio Macri, won 45.5 percent of ballots cast.

He needed to break the 50-percent mark in order to avoid a second round of voting, which will take place July 19.

Still, his relatively strong showing was positive for Macri, who will have to carry the city by a wide margin if he is to win the presidency in the October general election. Buenos Aires accounts for about 8 percent of Argentina's national vote.

In second place, according to the early results on Sunday, was Martin Lousteau with 26.9 percent of the vote. The former economy minister represents the ECO party, which is also opposed to outgoing President Cristina Fernandez.

In third place was Mariano Recalde, who heads state-controlled airline Aerolineas Argentinas and has been endorsed by Fernandez. He got 21.5 percent of the early-counted votes.

Fernandez is constitutionally barred from seeking a third consecutive term in October. For president she has endorsed Buenos Aires Governor Daniel Scioli, who is running ahead of Macri in the opinion polls.

(Reporting by Jorge Otaola; Editing by Richard Chang)