With 288 out of the 395 seats up for grabs awarded, the party had captured 80 seats in Friday's election, Interior Minister Taib Cherkaoui told a news conference.
That is nearly double the 45 seats won by Prime Minister Abbas el Fassi's Independence Party which finished second and has headed a five-party coalition government since 2007.
The interior ministry was to release final results on Sunday.
"We thank the Moroccans who voted for the PJD and we can only be satisfied," PJD secretary general Abdelilah Benkirane told reporters.
Cars honked their horns while passengers threw fliers outside of car windows bearing images of a lamp, the party's symbol, in Morocco's seaside capital Rabat after the partial results were released.
According to a new constitution overwhelmingly approved in a July referendum, King Mohammed VI must now pick the prime minister from the party which won the most seats in parliament, instead of naming whomever he pleases.
The king, the latest scion of a monarchy that has ruled the country for 350 years, proposed changes to the constitution that curb some of his near absolute powers as autocratic regimes toppled in nearby Tunisia, Egypt, and Libya and pro-democracy protests brewed at home.
The Islamists will have to govern with other parties and Benkirane acknowledged his party would have to tailor its programme to appease prospective coalition partners.
The PJD was "open to everyone" when it comes to forming alliances, he said.
"The nub of our programme and of those who will govern with us will have a double axis, democracy and good governance," Benkirane told the France 24 television channel.
The PJD has gradually increased its share of the vote in Morocco, seen as one of the most stable countries in the region.
After winning just eight seats in 1997, it surged in popularity, scooping 42 seats in the 2002 election, the first of King Mohammed VI's reign.
It then increased its share in the last election in 2007 when it finished second with 47 seats.
The party focused at first on social issues, such as opposition to summer music festivals and the sale of alcohol, but has shifted to issues with broader voter appeal like the fight against corruption and high unemployment.
During the current campaign it promised to cut poverty in half and raise the minimum wage by 50 percent.
Unlike the banned Islamist opposition group Justice and Charity, the Justice and Development Party pledges its allegiance to the monarchy.
But Benkirane cuts a controversial figure for previously derogatory comments about Berber people and homosexuals.
In 2010 he tried to ban a concert in Morocco by the openly gay singer Elton John because of fears of encouraging homosexuality.
Benkirane said Western nations, on whose investment and tourism the country relies heavily, had no reason to fear a PJD-led government.
"With the PJD there will never be surprises. We are going to develop relations with the West," he told AFP.
Two parties that make up the outgoing governing coalition -- the Independence Party and Socialist Union of Popular Forces - have said they would be willing to govern with the Islamist party.
The king proposed a new constitution on March 9, just 17 days after thousands of people took to the streets across Morocco calling on him to give up some of his powers in the biggest anti-establishment protests in the country in decades.
Morocco's pro-reform February 20 protest movement, responsible for the protests, had called on voters to boycott the elections. It says the constitutional reforms are insufficient.
Voter turnout was 45.4 percent, up from 37 percent from the last parliamentary election in 2007, but lower than the 51.6 percent turnout recorded in 2002.
El Fassi called the turnout "positive" given the election boycott calls.
"The consolidation of institutions and the promotion of democracy will contribute greatly in the future to improving the turnout rate," he added.
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That is nearly double the 45 seats won by Prime Minister Abbas el Fassi's Independence Party which finished second and has headed a five-party coalition government since 2007.
The interior ministry was to release final results on Sunday.
"We thank the Moroccans who voted for the PJD and we can only be satisfied," PJD secretary general Abdelilah Benkirane told reporters.
Cars honked their horns while passengers threw fliers outside of car windows bearing images of a lamp, the party's symbol, in Morocco's seaside capital Rabat after the partial results were released.
According to a new constitution overwhelmingly approved in a July referendum, King Mohammed VI must now pick the prime minister from the party which won the most seats in parliament, instead of naming whomever he pleases.
The king, the latest scion of a monarchy that has ruled the country for 350 years, proposed changes to the constitution that curb some of his near absolute powers as autocratic regimes toppled in nearby Tunisia, Egypt, and Libya and pro-democracy protests brewed at home.
The Islamists will have to govern with other parties and Benkirane acknowledged his party would have to tailor its programme to appease prospective coalition partners.
The PJD was "open to everyone" when it comes to forming alliances, he said.
"The nub of our programme and of those who will govern with us will have a double axis, democracy and good governance," Benkirane told the France 24 television channel.
The PJD has gradually increased its share of the vote in Morocco, seen as one of the most stable countries in the region.
After winning just eight seats in 1997, it surged in popularity, scooping 42 seats in the 2002 election, the first of King Mohammed VI's reign.
It then increased its share in the last election in 2007 when it finished second with 47 seats.
The party focused at first on social issues, such as opposition to summer music festivals and the sale of alcohol, but has shifted to issues with broader voter appeal like the fight against corruption and high unemployment.
During the current campaign it promised to cut poverty in half and raise the minimum wage by 50 percent.
Unlike the banned Islamist opposition group Justice and Charity, the Justice and Development Party pledges its allegiance to the monarchy.
But Benkirane cuts a controversial figure for previously derogatory comments about Berber people and homosexuals.
In 2010 he tried to ban a concert in Morocco by the openly gay singer Elton John because of fears of encouraging homosexuality.
Benkirane said Western nations, on whose investment and tourism the country relies heavily, had no reason to fear a PJD-led government.
"With the PJD there will never be surprises. We are going to develop relations with the West," he told AFP.
Two parties that make up the outgoing governing coalition -- the Independence Party and Socialist Union of Popular Forces - have said they would be willing to govern with the Islamist party.
The king proposed a new constitution on March 9, just 17 days after thousands of people took to the streets across Morocco calling on him to give up some of his powers in the biggest anti-establishment protests in the country in decades.
Morocco's pro-reform February 20 protest movement, responsible for the protests, had called on voters to boycott the elections. It says the constitutional reforms are insufficient.
Voter turnout was 45.4 percent, up from 37 percent from the last parliamentary election in 2007, but lower than the 51.6 percent turnout recorded in 2002.
El Fassi called the turnout "positive" given the election boycott calls.
"The consolidation of institutions and the promotion of democracy will contribute greatly in the future to improving the turnout rate," he added.
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