Human rights groups have condemned Nigeria for hosting Sudan's President Omar al-Bashir, and have demanded his arrest on genocide charges.
Mr Bashir is attending a health summit convened by the African Union (AU) in the capital, Abuja.
His visit was an "affront to victims" of the conflict in Darfur, rights groups said.
The International Criminal Court (ICC) accuses him of committing genocide in Darfur, a charge he denies.
The African Union decided in 2009, soon after the arrest warrant was issued, that member states should not enforce it.
It accuses the ICC of complicating peace efforts in the region, and unfairly targeting Africans.
The BBC's Chris Ewokor in Abuja says Mr Bashir received a full guard of honour when he landed in Abuja on Sunday to attend the summit, which will focus on tackling malaria, HIV and tuberculosis in Africa.
New York-based campaign group Human Rights Watch said Nigeria had the "shameful distinction" of being the first West African state to welcome Mr Bashir since the arrest warrant was issued.
Its decision was an "affront to victims" of the Darfur conflict, it added.
"He belongs in custody," said Elise Keppler at Human Rights Watch.
Nigeria's government had breached its obligations under international law by inviting Mr Bashir, said Chino Obiagwu, chair of the Nigerian Coalition for the International Criminal Court (NCICC).
"It sends a very wrong signal to other African countries that the International Criminal Court could be ignored... that African nations don't need to co-operate with the ICC and if that impression continues, then we're going to have serious problems dealing with impunity in Africa," he told the BBC's Focus on Africa programme.
Some 2.7 million people have fled their homes since the conflict began in Darfur in 2003, and the UN says about 300,000 have died - mostly from disease.
Sudan's government says the conflict has killed about 12,000 people and the number of dead has been exaggerated for political reasons.
Mr Bashir has visited numerous African countries since the arrest warrant was issued - including Ethiopia, Kenya and Djibouti.
Only Botswana and Malawi have threatened to arrest him.
In May, the AU called on the ICC to drop war crimes charges against Kenya's President Uhuru Kenyatta after accusing it of "hunting" Africans because of their race.
The ICC refused, saying it would press ahead with its case against Mr Kenyatta.
He is accused of fuelling violence after Kenya's 2007 election - a charge he denies.