ABUJA (Reuters) - As many as 39 people have been killed in election violence in Nigeria, civil society groups said on Sunday, as the country awaited the results of voting in what was forecast to be its tightest election since the end of military rule in 1999.

The Situation Room - which represents more than 70 civil society groups - gave the death toll the day after voting, citing data from Lagos-based consultancy SBM Intelligence. But authorities were braced for the possibility of further unrest after election results emerge later this week.

President Muhammadu Buhari, 76, a former military ruler who is seeking a second term on an anti-corruption platform, faces Atiku Abubakar, 72, a businessman and former vice president who has promised to expand the role of the private sector.

Deputy Inspector General of Police Abdulmajid Ali said police were still gathering information and could not provide figures on casualties. He said violence was greatest in the southern states of Rivers and Akwa Ibom.

The Nigeria Police Force said 128 people had been arrested nationwide for election-related offences including homicide, theft of ballot boxes and vote fraud. Thirty-eight weapons and a cache of explosives were also recovered.

The Situation Room said the worst incident was in Abonnema in Rivers State, around 14 km (9 miles) west of the main oil industry city of Port Harcourt, where the army said seven people died in a shoot-out between an unidentified gang and Nigerian soldiers.

A lieutenant and six gunmen died in the firefight, which broke out when the attackers barricaded a major road into the town and ambushed the troops, the acting director of army public relations, Colonel Sagir Musa, said. Their motives were not known.

Nigerian security forces are stretched by an Islamist insurgency in the northeast along with communal violence and banditry in other areas. Previous elections have been marred by violence among supporters of different political parties that at times sparked sectarian clashes.

A credible and relatively calm poll would open a new chapter in the checkered political history of Nigeria, where nearly six decades of independence have been tarnished by military coups, endemic corruption and secessionist movements.

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The number of deaths reported Sunday was below the final tolls in previous elections in Africa’s most populous country. However, most unrest typically occurs after results are announced.

Results are expected to start trickling in early on Monday with a “substantial number” likely on Tuesday, election commission official Festus Okoye said.

As many as 100 people were killed during and after the last election in 2015, according to the International Crisis Group.

Four years before that, in the religiously mixed northern city of Kaduna, 800 people died when fighting erupted between Muslims and the mainly Christian backers of then-President Goodluck Jonathan.

Islamist militants in the Northeast vowed to disrupt this year’s election. Hours before polls opened, explosions rang out in Maiduguri, capital of Borno state at the epicenter of the decade-long insurgency. In neighboring Yobe state, residents of Geidam town fled an attack around the same time.

The election was postponed the previous Saturday just hours before it was due to begin, with the authorities citing problems in delivering ballot papers and results sheets in some parts of the country.

VOTING EXTENDED IN A FEW PLACES

Despite the violence, some noted that it could have been worse.

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“We didn’t expect that the election would be so smooth ... we thought people might be dying,” said Henry Okalome, 55, a trader in Kano who voted for Atiku.

Violence and problems with electronic voting machines meant voting took place in a small number of places on Sunday.

Seventy-three million eligible voters chose from a pool of more than 70 presidential candidates, although only those from the two main parties - Buhari and Atiku - are seen as having a chance.

The contest between the two men hinges on revamping an economy struggling to recover from its first recession in 25 years, which it slipped into in 2016 and emerged from in 2017.

Nearly a quarter of the workforce is unemployed, while the cost of living has also risen rapidly.

Buhari has focused his campaign on rooting out corruption, but critics say there have not been any significant convictions in his first term. Atiku has said he would aim to double the size of the economy to $900 billion by 2025.