Sheikh Ali Salman, the leader of the Islamic National Accord Association (INAA) and a candidate in the mostly Shia northern town of Jid Hafas, complained about queues, saying "there should be more ballot boxes".
However, he said that the high turnout "proves the popularity" of the INAA, the main Shia opposition grouping.
"There should be more ballot boxes"
Sheikh Ali Salman, the leader of the INAA
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Salman predicted that the association would take "at least 13 seats in the first round" and that all 17 candidates should win after the second round of polling on December 2.
He said: "If this doesn't happen, [we would assume that] it will be because of fraud," adding that "we will study each case separately".
During the election the main Shia grouping allied itself with Sunni liberals and left-of-centre candidates.
Like the Shias, they boycotted the last legislative elections in 2002 in protest at the split of legislative powers between parliament and an equally numbered upper chamber appointed by the king which can block parliamentary initiatives.
Many Shias accuse the government of plotting to maintain pro-government Sunni domination of the tiny kingdom, mainly through naturalised Bahrainis, including those coming from Saudi Arabia across the causeway linking the two countries.
The causeway's public voting centre is one of 10 out-of-constituency ballot stations which the opposition had demanded be shut.
Several Saudi registered cars were seen parked outside the polling station, and an INAA representative at the centre said many dual citizens came from Saudi Arabia just to vote.
On Friday some 2,000 demonstrators called for a probe into an alleged plot aimed at marginalising the Shia majority and demanded the resignation of Sheikh Khalifa bin Salman al-Khalifa, the country's long-serving prime minister.