More than 206,650 voters are said to have registered to choose among 800 candidates running in the eastern province, home of the Shia minority in the country.
In the south, about 115,000 voters have signed up and more than 1700 candidates are running in the second phase of a three-round election for municipal councils across the country.
Women, who make up more than 50% of the population, have been excluded from the polls, which are only open to men over 21. Military personnel are also not allowed to vote.
Women are also barred from participating in the Shura (consultative) Council's new session, due to start in two months.
Saleh bin Abd Allah bin Humaid, chairman of the Shura Council, told a news conference in Riyadh last month that the topic of women's participation as members in the council was not officially on the table.
Women, however, were promised to be allowed to play a role in the next municipal polls in 2009.
While men queued up to cast their ballots at a school that was turned into a polling station in al-Ahsa city in the southern part of the eastern province, a candidate found out his name had been removed from the candidates list.
"I am really shocked. I still don't know why I was excluded"
Husain Abd al-Rahman al-Khamis, Saudi candidate
Husain Abd al-Rahman al-Khamis, a Saudi candidate, told Aljazeera he was surprised and did not receive any notice of his removal.
"We were shocked when we went to poll stations on Thursday morning to find a piece of paper at some polling stations, like in al-Muthanna school ... that reads 'Husain Abd al-Rahman al-Khamis, candidate No.10155010, has been excluded from the candidates list of the first electoral district, according to a decision by contestation committee No.6/1' ... one day before the elections," al-Khamis told Aljazeera.
He said he was the only candidate removed from the list.
"I still don't know why I was excluded. People to vote for me are also shocked," he said, adding the paper had no official stamp and vowing to take the issue to court.
Attempt to reform
Voters in the western regions of Mecca and Medina, as well as in the northern regions, will go to the polls on 21 April in the third and final round of the elections, which are the first nationwide ballot since the 1960s.
About 30% of eligible voters took
part in the first round last month
The first round in the capital, Riyadh, on 10 February saw 73.6% of 140,000 registered voters - about 30% of total eligible voters - take part.
The elections are to pick half the members of 178 local councils across the oil-rich country. The other half will be appointed by the government.
The municipal first-round vote last month came a week after US President George Bush urged Riyadh to "demonstrate its leadership in the region by expanding the role of its people in determining their future".
The nationwide vote is said to be a cautious initial step towards reforms in the ultra-conservative kingdom, which Riyadh insists must be tailored to Saudi specifications and not necessarily follow a Western pattern.