Nigerian police said on Tuesday they had arrested more than 90 demonstrators carrying Russian flags at protests triggered by economic hardship.
Thousands of people joined protests against government policies and the high cost of living last week as Africa's most populous country suffers its worst economic crisis in a generation.
The rallies have petered out in most parts of the country following clashes with security forces, but hundreds of protesters took to the streets in northern states on Monday including Kaduna, Katsina and Kano, as well as central Plateau state.
AFP journalists and witnesses saw some demonstrators holding Russian flags, a development the Russian embassy distanced itself from.
Northern Nigeria shares strong cultural, religious and socioeconomic ties with neighbours in the Sahel region, which has seen a string of coups and military leaders turning away from Western allies towards Russia.
Russian flags have featured at rallies in Niger, Mali and Burkina Faso, and their appearance in Nigeria triggered strong reactions from officials.
Asked about the protesters in Nigeria, police spokesman Olumuyiwa Adejobi said "we have more than 90 of them arrested with the Russian flags."
Later on Tuesday, security chiefs from the armed forces and police among others gave a rare joint briefing and alleged unnamed "sponsors" were seeking to undermine the government, without providing evidence.
"The sponsors of these protests, some of them, have a clear motive to subvert the government of the day, we are not going to allow that, we will defend our democracy," said police chief Kayode Egbetokun.
He alleged that some sponsors were "outside the country" and said "we have to arrest those carrying flags to be able to get to the sponsors".
On Monday, defence chief General Christopher Musa said those pushing individuals to carry Russian flags in Nigeria were "crossing the red line and we will not accept that".
Damilare Adenola, leader of the Take It Back group organising protests in Abuja, dismissed the allegations as "mere distraction".
He said the authorities were using the claims as a "reason to clamp down on protesters".
The Russian embassy in Nigeria denied involvement in a statement on its website on Monday.
"The Government of the Russian Federation as well as any Russian officials are not involved in these activities and do not coordinate them in any way," it said.
Rights group Amnesty International has accused security forces of killing at least 13 demonstrators in the first day of protests on Thursday, while police say seven people have died and denied responsibility.
In a televised address on Sunday, President Bola Ahmed Tinubu called for the suspension of demonstrations -- but protest organisers have vowed to press ahead despite lower turnout.