The European Union Observation Mission to Nigeria says soldiers barred them from monitoring elections in Rivers State.
The EU Mission also observed that notable lack in equality of opportunity in the state's election, is dangerous for democracy.
The observers made the allegation in their preliminary report in Abuja on Monday.
“Observers, including EU observers, were denied access to collation centres in Rivers, apparently by military personnel. This lack of access for observers compromises transparency and trust in the process, said head Maria Arena.
She noted that they came to Nigeria on the invitation of INEC to monitor the elections and make recommendations.
Arena, who is a member of the EU Parliament, however, faulted the elections.
“In Rivers, INEC suspended until further notice the elections due to violence in polling units and collation centres, staff being taken hostage and election materials, including results sheets, seized or destroyed by unauthorised persons.
“There is no doubt that the electoral process there was severely compromised.”
Arena lamented the violence, underage voting and vote-buying that marred the governorship and House of Assembly elections in some states.
She said the EU deployed 73 observers following voting, counting and the collation of results in 22 states.
The chief observer said they monitored 223 polling units in 81 collation centres.
She stressed the need for electoral reforms in the country, adding that the problems evident in the 2019 electoral process had shown that there was a need for an inclusive national discussion on reforms for greater electoral integrity and participation.
Arena also noted misuse of power of incumbency by the governors of the All Progressives Congress and Peoples Democratic a Party on state-owned media houses.
Documented reports of large-scale human rights abuses by the Nigerian Military have shadowed the country for years, often straining relations with international partners, including the United States, in the fight against Boko Haram.
The abuses constitute war crimes and possible crimes against humanity, the Amnesty International contended in its 133-page report.