SAO PAULO (Reuters) - Brazil's biggest private bank, Bradesco (BBDC4.SA: Quote, Profile, Research)(BBD.N: Quote, Profile, Research), said on Thursday it would buy the country's top brokerage, Agora, in an all-stock transaction worth about 830 million reais ($494 million).

After the deal is completed, Agora shareholders will end up with about 8 percent of the capital of Bradesco's investment banking unit, BBI, and Agora will become a BBI subsidiary, Bradesco said in a statement.

Out of the total sum of the transaction, 500 million reais is the value to be paid for Agora itself and 330 million reais correspond to the market value of shares in Brazil's key bourses, Bovespa and BM&F, held by Agora, the bank said.

Bradesco first acknowledged it was in talks to buy the 15-year-old Rio de Janeiro-based brokerage house in late January.

"The acquisition will allow Bradesco to take over the leadership in a segment characterized by high growth rates and big competition," Bradesco said, adding that the deal was still subject to regulators' approval and results of due diligence.

Agora's online "home broker" service for private individuals has 29,000 active clients, Bradesco noted.

Bradesco's shares in Sao Paulo fell 4.5 percent prior to the announcement, which was made after the closing bell.

($1=1.68 reais)

(Reporting by Andrei Khalip and Alexandre Caverni; Editing by Braden Reddall)