CPP officials said the party won at least 68 seats, more than the 55 seats won by the main opposition, the BBC reported.
Official results were expected later Sunday.
More than 9 million people were eligible to vote in the elections. Observers predicted Prime Minister Hun Sen, of the CPP, would be re-elected, extending his 28-year rule, Voice of America reported.
Opposition leader Sam Rainsy of the Cambodia National Rescue Party has alleged there were voting irregularities and vowed to hold protests should there be any evidence of fraud.
However, the national election committee said that it worked hard to ensure a fair voting day, CNN reported.
"In preparing for the election this year, we started in the middle of 2012," Tep Nytha, secretary-general of the committee, said before voting started.
Both Sen, 60, and Rainsy, 64, were seen casting their votes early on Sunday, the BBC reported.
Rainsy was greeted by enthusiastic voters at a polling station near his party headquarters in Phnom Penh.
"I am happy to see people flocking to vote," he said.