Speaking to reporters, Kadhimi urged voters to head to polling stations.
"Go out and vote, change your reality," Kadhimi said.
The election is being held several months early under a new law designed to help independent candidates - a response to mass anti-government protests two years ago. But the established, armed and Shi'ite Islamist-dominated ruling elite is expected to sweep the vote.
At least 167 parties and more than 3,200 candidates are competing for Iraq's 329 seats in parliament, according to the country's election commission.
Iraqi elections are often followed by months of protracted negotiations over a president, a prime minister and a cabinet.
Polls opened at 7 a.m. (0400 GMT) and will close at 6 p.m. (1500 GMT).
In 2019, mass anti-government protests swept across Baghdad and the south, toppled a government and forced Kadhimi's current government to hold this election six months early.