The 18 candidates of INAA, which clinched 17 seats at the last poll in 2006, were all elected from a first round, with more than the required 50 percent of votes, commission chairman Abdullah al-Buainain told AFP.
Top Shiite cleric and MP Sheikh Ali Salman hailed the results and called for a "more positive" stance from the government.
"The most important message for the government is that Al-Wefaq (INAA) is the largest political association in Bahrain," said Salman, who is also the head of INAA.
"The people's will must be respected and dealt with positively."
Before the close of campaigning, Salman openly challenged the pro-Western Al-Khalifa family, a dynasty which has ruled Bahrain since 1783, saying that authority should be shared.
Bahrain's current government has several Shiite ministers but none of them are INAA members.
Reforms passed in a 2001 referendum restored a parliament dissolved in 1975 and turned the emirate into a constitutional monarchy, but Hamad's uncle, Prince Khalifa bin Salman al-Khalifa, has served as prime minister ever since independence from Britain in 1971.
On Saturday, Prince Khalifa pledged to cooperate with the legislative authority but at the same time dismissed the presence of an opposition political party in the country.
Nine seats remain up for grabs in a second round of voting on October 30.
Two candidates from the National Democratic Action Association, an alliance of pan-Arab nationalists and leftists which failed to win seats in 2006, including a woman, Munira Fakhru, are to run in the second round.
Eight women figured among the 127 candidates running in the first round.
Candidates of two allied Sunni Islamist groups, the National Islamic Forum and Al-Assalah, will also contest next Saturday's vote.
The Forum, the local arm of Egypt's Muslim Brotherhood, had seven seats in the outgoing parliament, and three of its eight candidates lost this time round, with the other five to run again on Saturday.
Al-Assalah, which had five seats, won two from the first round and is to contest another three on Saturday.
Some people complained that their names had been missing from voters' lists on Saturday, but senior officials dismissed the claims.
Justice Minister Sheikh Khaled bin Ali al-Khalifa, head of the electoral commission, estimated turnout of "at least 67 percent," compared with 72 percent in 2006 and 53.4 percent in 2002.
With Sunday's results, INAA strengthens its presence in the lower house of parliament which has the authority to examine and pass legislation proposed by the king or cabinet and also has monitoring powers.
A 40-member upper chamber, or consultative council, appointed by the king has the power to block legislation coming out of the lower house.
Ahead of the polls, a wave of arrests of Shiite political activists drew warnings from international human rights watchdogs of a drift back to full-blown authoritarianism.
According to analysts, the press campaign that accompanied the arrest of 23 Shiite activists on terror charges in September has come in favour of INAA.
"The Shiites who were hesitant or intended to boycott the elections voted overwhelmingly in favour of Al-Wefaq, the representative of their community," analyst Obaidaly al-Obaidaly told AFP.
Bahraini authorities detained 250 Shiite opposition activists, London-based watchdog Amnesty International said earlier this month.
"The way the media handled the security situation which prevailed prior to the elections unexpectedly raised Al-Wefaq's shares" in the votes, said Baqer al-Najar, a sociology professor at the University of Bahrain.
"Shiites felt that they were targeted so they voted intensely for Al-Wefaq despite their restlessness with its performance throughout the past four years," Najar said.
Foreign Minister Sheikh Khaled bin Ahmad al-Khalifa insisted on Saturday that the arrests were "not linked to elections."
The archipelago state was plagued in the 1990s by a wave of Shiite-led unrest which has abated since the 2001 reforms.
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