SINGAPORE, Aug 11 (Reuters) - Singapore will hold a presidential election on Sept. 1 if more than one candidate is nominated to run for the largely ceremonial post, the government said on Friday.

If there is only one candidate on nomination day, Aug. 22, he or she will be declared president-elect.

This year's presidential election in the diverse city-state will be open to all ethnic groups, unlike the last time, in 2017, when the post was controversially reserved for those of Malay ethnicity.

The 2017 election was uncontested when applications from four candidates were rejected, sparking a rare protest by several hundred people in a city park who raised a banner with the message "ROBBED OF AN ELECTION #NotMyPresident".

The current president, Halimah Yacob, Singapore's first woman president, has said she has chosen not to run for re-election. Her term ends on Sept. 13.

Three prominent figures have announced an interest in the post: former politician Tharman Shanmugaratnam, Ng Kok Song, who was the former chief investment officer at sovereign wealth fund GIC, and businessman George Goh.

They will have to get certificates of eligibility from the Presidential Elections Committee to stand.

In 2017, parliament amended the constitution so that the presidency would be reserved for a member of a particular ethnic group if no one from that group had been president for five six-year terms.

Singapore has about 3.5 million citizens, about three-quarters of them ethnic Chinese, 12.5% ethnic Malay and 9% ethnic Indian, with the rest classified as Eurasians.

As part of the 2017 amendment, eligibility requirements for the presidency were tightened so candidates must be a senior civil servant or have served as the chief executive of a company with paid-up capital of at least S$500 million ($371 million).

About 1,200 companies have that amount of shareholders’ equity and about 50 civil servants meet the criteria, according to a parliamentary response in May.