Marca surveyed members as the voting proceeded, with the figures covering the sample available at around 2pm local time. The lead for the incumbent is not unexpected, though some observers noted that Riquelme's share of the vote was running higher than the most pessimistic pre-election estimates had suggested.
Polling stations are open from 9am to 8pm local time, after which counting will begin. Results are expected between 10pm and 11pm CEST. Postal voting, particularly from members living outside Madrid, is expected to be a factor in the final count.
Pérez, 79, is seeking a seventh presidential term after calling a snap election in May following a second consecutive trophy-less season.
Riquelme, 37, announced his candidacy on May 21 and ran an aggressive campaign centred on opposition to any structural changes to club ownership, promising high-profile signings and backing from former club legends Vicente del Bosque, Raúl González Blanco, Fernando Hierro and Iker Casillas.
These are the first contested elections at Real Madrid since 2006, when Ramón Calderón defeated Pérez in a vote that led to the current president's only interruption in charge. Approximately 95,000 to 100,000 members are eligible to participate.