Barry Andrews, European Union Observer Mission Nigeria chief observer told a news conference that Lagos, which re-elected the ruling party's governor, was among several southern and central states that witnessed poll related violence.
"It's actually tragic that 21 people, according to our latest information, lost their lives in election-related violence," Andrews said.
"Polling on election day was again disrupted by multiple incidences of thuggery, intimidation of voters, polling officials, observers and journalists and this occurred in most of the states," he added.
Nigerians voted for state governors in 28 of 36 states on Saturday. Results were still expected from several states on Monday, including northeastern Adamawa, a conservative and Muslim state, which could elect its first female governor.
Andrews said the violence and intimidation may have led to a low turnout. While there is no nationwide turnout figure, because the voting is administered state by state, authorities put it at 16% in Lagos state - Nigeria's most populous with more than 20 million people.
He said, however, there were some improvements in Saturday's vote compared to last month's disputed presidential poll. Election materials arrived on time at most polling units, which opened early while electronic voting machines worked better than last month.
The number of reported deaths was below the final tolls in previous elections in Africa's most populous country. Most unrest typically occurs after results are announced.
