ADDIS ABABA, Ethiopia (AP) — Ethiopia was voting Monday in the greatest electoral test yet for Prime Minister Abiy Ahmed as insecurity and logistical issues meant ballots wouldn't be cast in more than 100 constituencies of the 547 across the country.

The election, delayed from last year, is the centerpiece of a reform drive by Abiy, whose rise to power in 2018 seemed to signal a break with decades of authoritarian rule and led to his Nobel Peace Prize the following year. He has described the poll as “the nation’s first attempt at free and fair elections.”

Long lines of voters were seen in some parts of the capital, Addis Ababa, while security was stepped up across Africa's second most populous country. Military vehicles were parked in key locations in the capital. More than 37 million Ethiopians were expected to vote.

Abiy’s ruling Prosperity Party, formed in 2019 by merging groups who made up the previous ruling coalition, is widely expected to cement its hold on power. The party that wins a majority of seats in the House of Peoples’ Representatives will form the next government.

Opposition groups have accused Ethiopia’s ruling party of harassment, manipulation and threats of violence that echo abuses of the past. Some prominent opposition parties are boycotting the election. Others say they were prevented from campaigning in several parts of the country.

Abiy is facing growing international criticism over the war in Ethiopia’s northern Tigray region, sparked in part because Tigray's now-fugitive leaders objected to Ethiopia postponing the election last year while citing COVID-19. No date has been set for voting in Tigray’s 38 constituencies.

Tigray’s former leaders, who are fighting Ethiopian forces and those from neighboring Eritrea, have reported fierce new combat in recent days. Ethiopia’s defense forces have called the fighting challenging because of the rough terrain. Thousands of civilians have been killed in what observers describe as drawn-out guerrilla war.

Meanwhile, outbreaks of ethnic violence have killed hundreds of people in the Amhara, Oromia and Benishangul-Gumuz regions in recent months.

International concern has been growing about the election. The U.S. has said it is “gravely concerned about the environment under which these upcoming elections are to be held," and the European Union said it will not observe the vote after its requests to import communications equipment were denied.

In response, Ethiopia said external observers “are neither essential nor necessary to certify the credibility of an election,” although it has since welcomed observers deployed by the African Union.

The United Nations secretary-general has noted the “challenging” environment and warned against acts of violence.

“It is our duty to remain united and not the government’s,” one resident of the capital, Eskedar Teklegiorges, said over the weekend as hundreds of police officers paraded in a show of force ahead of the vote.

Abiy’s Prosperity Party registered 2,432 candidates in the election. The next largest party, Ethiopian Citizens for Social Justice, was fielding 1,385 candidates. A total of 47 parties were contesting.