The single-round ballot will decide the 165 seats of Senegal's unicameral parliament -- currently controlled by the president's supporters -- for the next five years.
For the government, the parliamentary polls are a second test following municipal elections in March, which the opposition won in major cities, including the capital Dakar, Ziguinchor in the south and Thies in the west.
Eight coalitions are in the running for these elections, including Yewwi Askan Wi (meaning "Liberate the People" in Wolof), the main opposition coalition.
Its highest-profile member, Ousmane Sonko, came third in the 2019 presidential election. But he has been banned from running in Sunday's elections.
That coalition has joined forces with Wallu Senegal (which means "Save Senegal" in Wolof), led by former president Abdoulaye Wade.
The two groups have agreed that whichever one places lower in each department will support the other, in order to obtain a parliamentary majority and "force governmental cohabitation."
The opposition also wants to force Sall to give up any hope of running in 2024.
"If Macky Sall loses, he won't be talking about seeking a third term," Sonko said following protests in June.
"It's the first round of the 2024 presidential election," researcher and political analyst Cheikh Gueye told AFP.
The election is taking place against a backdrop of rising food and fuel prices following Russia's invasion of Ukraine.
The opposition says the government has been unable to address the high cost of living.
But the latter says it has introduced subsidies to help people cope with the price hikes, and has a good track record of investing in infrastructure.
The 21-day election campaign, during which giant rallies and noisy caravans criss-crossed the country in a mostly calm atmosphere, ends on Friday evening.
The pre-campaign period, however, was marked by violent demonstrations that left at least three people dead.
The violence came after the interior ministry tossed out a list of Yewwi Askan Wi's first-choice election candidates, citing technical grounds. The country's highest court backed the decision.
One of the names had been accidentally put down both as a first-choice candidate and as an alternate candidate, thus invalidating the entire list.
The move banned Sonko and others from participating in the elections.
Senegal has a general reputation for stability in a region where political turbulence is widespread.
But the country was shaken by several days of rioting in March last year, claiming around a dozen lives, after Sonko was accused of rape.