Polling stations opened across France at 8:00 am (0600 GMT) and voting will continue in the largest cities until 8:00 pm (1800 GMT) with exit polls released immediately afterwards.
Macron has enjoyed a smooth start in the five weeks since he beat far-right candidate Marine Le Pen to become France's youngest-ever president, naming a cabinet that crosses left-right lines and making a big impression at international summits.
His untested Republique en Marche (Republic on the Move, REM) party, which he only founded in April 2016, now needs a clear majority in the National Assembly for him to push through the reforms he has promised.
A host of opinion polls show Macron's party taking around 30 percent of the vote on Sunday, putting it in pole position to secure a landslide in the second round next Sunday.
Some predictions show REM could take around 400 seats in the 577-seat chamber. The party has already had a boost after its candidates came first in 10 of the 11 French overseas constituencies that voted before the mainland.
If no candidate wins over 50 percent in the first round, the two top-placed go into the second round – as well as any candidate who won the votes of over 12.5 percent of the electorate.
More than 50,000 police will be on patrol in a country still under a state of emergency following a wave of attacks that have killed more than 230 people since 2015.
In the latest incident, which took place on Tuesday, a 40-year-old self-radicalised Algerian was shot and wounded after attacking a policeman with a hammer outside Paris' Notre Dame cathedral.
Breaking the mould
French voters have traditionally rallied behind their new leader in the legislative elections that follow the presidential ballot.
Macron's predecessors Francois Hollande in 2012, Nicolas Sarkozy in 2007 and Jacques Chirac in 2002 all won outright majorities. Unlike Macron, however, they all came from long-established parties.
REM reflects the president's desire for a new brand of politics.
Initially dismissed by Macron's opponents as a movement of young activists without any real roots, it will field 530 candidates on Sunday.
In a bid to renew the political scene, many have never stood for office before, such as Marie Sara, a rare female bullfighter, who is taking on a senior member of Le Pen's National Front in southern France, Gilbert Collard.
Cleaning up politics
Macron has banned all REM candidates from employing family members if elected and they must not perform consultancy work while lawmakers.
The edicts follow the scandal that sunk the presidential chances of Francois Fillon, candidate for the rightwing Republicans party, who is facing criminal charges for paying his wife Penelope more than €900,000 ($1.0 million) as his parliamentary assistant.
Fillon denies the charges.
Given Macron's attempts to clean up French politics, he faced embarrassment on Friday when his small centrist ally, the MoDem party, was placed under preliminary investigation on suspicion of employing fake parliamentary assistants at the European Parliament.
One of Macron's ministers, Richard Ferrand, is also being probed over a property deal involving his wife.
With the political tide turning against the main parties of left and right, they have warned that a landslide could be bad for democracy.
“I don't think it would be healthy for the democratic debate over the next five years,” said Francois Baroin, who is leading the Republicans as they try to bounce back from Fillon's failure in the presidential election.
The Socialists of former president Hollande fear heavy losses after a disastrous performance in the presidential election.
Le Pen defiant
Le Pen's party meanwhile looks set to struggle to win 15 seats nationally, a score that would represent another deep disappointment after she was soundly beaten by Macron.
But Le Pen remains defiant, telling AFP this week that with other parties likely to agree to work with Macron, “we will be the only opposition force.”
Macron has appealed to voters to give him a strong mandate to overhaul the labour market whose rigid rules on hiring and firing are blamed by many economists for preventing growth.
The president was economy minister in the previous Socialist government that began introducing the reforms, sparking mass demonstrations in 2016 that lasted for months.
By Sylvie Groult and Guy Jackson