Lagos, Nigeria (CNN) Multiple bomb blasts rocked the northern Nigerian city of Maiduguri just hours before presidential polls opened Saturday.

The explosions happened at a camp for internally displaced refugees at around 5 a.m. local time Saturday, Nigerian army spokesman Onyeama Nwachukwu told CNN. There were no reports of any deaths or injuries.

"There was an attack this morning at the camp by the militants, but the military has suppressed it at the moment," Nwachukwu said, adding that the army was still assessing the situation.

Elsewhere, police told CNN that three people were killed and one wounded in election-related violence in Rivers state in the southeast.

Nigerian voters wait for the scheduled opening of polling stations in Kaduna.

Nigerian voters wait for the scheduled opening of polling stations in Kaduna.

An election official hands a ballot to a voter at a polling station in Tumfafi.

An election official hands a ballot to a voter at a polling station in Tumfafi.

Electoral commission staff deliver items to a polling station in Port Harcourt after voting was due to begin.

Electoral commission staff deliver items to a polling station in Port Harcourt after voting was due to begin.

Women line up to cast their votes in the village of Tumfafi, near Kano.

Women line up to cast their votes in the village of Tumfafi, near Kano.

A staff member of the Independent National Electoral Commission marks the thumb of a voter at a polling unit in Lagos.

A staff member of the Independent National Electoral Commission marks the thumb of a voter at a polling unit in Lagos.

People line up to vote in the presidential election a few hours before polls open at Shagari Health Unit polling station in Yola.

People line up to vote in the presidential election a few hours before polls open at Shagari Health Unit polling station in Yola.

Presidential candidate and main opposition leader Atiku Abubakar casts his vote along with his wife, Amina Titilayo Atiku-Abubakar, at Agiya polling station in Yola.

Presidential candidate and main opposition leader Atiku Abubakar casts his vote along with his wife, Amina Titilayo Atiku-Abubakar, at Agiya polling station in Yola.

Nigeria's President, Muhammadu Buhari, gestures to supporters after casting his vote in his hometown of Daura.

Nigeria's President, Muhammadu Buhari, gestures to supporters after casting his vote in his hometown of Daura.

People line up to vote in Kaduna, Nigeria, on February 23.

People line up to vote in Kaduna, Nigeria, on February 23.

People peer through a window as party agents huddle together to watch votes being counted inside a polling station in Kano, Nigeria.

People peer through a window as party agents huddle together to watch votes being counted inside a polling station in Kano, Nigeria.

One of the first voters casts his ballots for presidential and parliamentary seats on Saturday, February 23, at the Unguwar Sarki polling station in Kaduna, Nigeria.

One of the first voters casts his ballots for presidential and parliamentary seats on Saturday, February 23, at the Unguwar Sarki polling station in Kaduna, Nigeria.

Journalist Simpa Samson told CNN the militants targeted the Teacher's Village camp in Maiduguri, the capital of Nigeria's Borno state.

"The military secured the place almost immediately and has stopped our cameraman from filming, no one was hurt because the bombs landed outside the premises," Samson told CNN.

Security is often a concern in Maiduguri, a frequent target of terror group Boko Haram. The city also has a large population of internally displaced refugees.

The blasts came as Nigerians prepared to cast their ballots Saturday, a week after the country's presidential and parliamentary elections were unexpectedly postponed. It was the third consecutive vote to be delayed in Africa's largest democracy.

Delays frustrate voters

Polls across the country were scheduled to open at 8 a.m. (2 a.m. ET) and close at 2 p.m. (8 a.m. ET) Saturday, but reports soon emerged of widespread delays. They officially closed at 3 p.m. (9 a.m. ET).

In the megacity of Lagos, CNN visited multiple polling stations, all of which opened hours later than planned. Voters said they had lined up for hours before electoral officers arrived with voting materials.

To cast their ballots Saturday, voters were expected to complete an accreditation process in which officials from Nigeria's Independent National Electoral Commission take their fingerprints and scan their permanent voting cards.

A nurse told CNN she turned up at a polling station after working a night shift, only to face a long wait.

"I am supposed to be resting now, but I came to the polling station (at) 7:30 a.m. thinking they the electoral commission officials) will be here, but it's two hours now, and they have not come," Juliet Emoedin said.

Festus Okoye, a national commissioner for the electoral commission, said stations that opened late would close an hour later, according to the Nigerian Television Authority, the state broadcaster.

However, a one-hour extension is less time than many voters have spent in line to cast their ballots in the crucial election.

The incumbent, Muhammadu Buhari, 76, is running against 71 other candidates, but his main challenger is Atiku Abubakar, a 72-year-old business tycoon and former vice president. They are both Muslim candidates from the north of the country.

When Buhari, a former military ruler, was elected in 2015, it was the first peaceful transition of power in Nigeria. He promised to offer a clean sweep of the old routine, but many have been left disillusioned and angry at the rising levels of inequality and extreme poverty.

Buhari appeared optimistic Saturday about his re-election chances when he cast his ballot in his hometown of Daura in the northern state of Katsina.

When asked if he would accept defeat if he lost and congratulate whoever won, Buhari said: "I will congratulate myself. I am going to be the winner."

Atiku Abubakar, the candidate of the main opposition People's Democratic Party, voted in Yola in Adamawa state.

Abubakar urged citizens to participate in the election.

"Go out and cast your vote too if you have not done so yet," Abubakar tweeted

The People's Democratic Party has complained that card readers used at polling stations have been slowing down voting.

"PDP calls on INEC to allow manual voting," party campaign spokesman Osita Chidoka wrote in a statement, "and to extend voting time to allow all voters with registered PVC (permanent voter card) to vote today."

More than 84 million people registered for the vote in Africa's largest economic power, according to data from the Independent National Electoral Commission.

Videos have surfaced on social media reportedly showing the burning of ballot papers and disruption of the electoral process in various parts of the country.

A woman glances over at the ballot boxes one last time before casting her vote Saturday in Lagos.

Concerns over election-related violence

In Rivers state, "hoodlums" fatally shot a soldier and injured another who foiled an attempt to steal ballot boxes at a polling station, police spokesman Nnamdi Omoni told CNN. Gunmen also killed a member of the ruling All Progressives Congress party and his brother in the state, police said.

In addition, Nigerian army said its troops had also repelled an attack Saturday morning by suspected militants on a security outpost in Geidam village in Yobe state.

No one was injured, according to Col. Sagir Musa of the Nigeria army, who said the attempt would not affect voting in the area.

"The situation is calm and peaceful," Musa said in a statement. "People have largely turned out to cast their votes without any hindrance."

The election delay has increased tensions in Nigeria, and there have been instances of violence in the lead-up to the vote. The British and US governments have warned they would deny visas to, and could prosecute, anyone found inciting violence during the election.

Last week, a terror group with links to ISIS claimed responsibility for a deadly attack in Maiduguri on a motorcade carrying Kashim Shettima, the governor of Borno state.

Shettima escaped unscathed. Isa Gusau, the governor's media aide, told CNN on Thursday that the ambush killed three people, although locals put the death toll much higher. The terror group claimed that 42 people died in the assault.

The Islamic State's West Africa Province terror group, a breakaway faction of the Boko Haram militant group, has staged a number of high-profile attacks in recent months.