A former leader of the Nigerian militant group Mend has been arrested in connection with two blasts on Friday in Abuja which killed 12 people.
Henry Okah was arrested in South Africa, said a spokeswoman for the Nigerian secret police, Marilyn Ogar.
However, Nigerian President Goodluck Jonathan said “terrorists” and not Mend were responsible for the explosions.
The State Security Service confirmed it had been warned by foreign intelligence services that an attack was imminent.
The Movement for the Emancipation of the Niger Delta (Mend), which says it is fighting for a fairer distribution of Nigeria’s oil wealth, said it carried out the car bombings not far from the official celebrations.
President Jonathan, who is himself from the Niger Delta, did not elaborate on who he believed carried out Friday’s bombings.
But Mr Okah, who leads a faction of Mend, told the BBC on Friday his group was not responsible.
The BBC’s Caroline Duffield in Lagos says Mr Okah has barely been heard of outside Nigeria but within the country, he is notorious.
Mend signed an agreement with the government last year, in which former fighters were offered an amnesty and small amounts of cash in return for handing in their weapons.
Mr Okah, known as the gunrunner of Mend, is widely believed to lead a splinter group opposed to the amnesty, our correspondent says.
His friends expect him to be charged on Monday.
Mend sent a warning shortly before the blasts, saying “several explosive devices have been successfully planted in and around the venue by our operatives working inside the government security services”.
It added: “There is nothing worth celebrating after 50 years of failure. For 50 years, the people of the Niger Delta have had their land and resources stolen from them.”
Mend later accused officials of acting “irresponsibly by ignoring our forewarning”.
British dignitaries stayed away from Friday’s parade marking 50 years of Nigerian independence, raising questions about how much foreign intelligence services knew about threats to the event.
The two bombs went off about five minutes apart. Police said the bombs appeared timed to do most damage to those who responded to the first blast.
A UK Foreign and Commonwealth Office spokesperson confirmed it had “received indications of a heightened security risk” and took immediate action to update travel advice after the Mend warning on Friday morning.
The Queen’s representative, the Duke of Gloucester, had been scheduled to attend the parade but did not do so. His office declined to comment on why he withdrew. Former British prime minister Gordon Brown also did not attend.
Since the Nigerian government and Mend signed an amnesty agreement last year, incidents of violence and kidnapping in the Delta have fallen.
However, many fighters complain the government has failed to deliver its end of the deal.
Oil production has increased since the amnesty came into effect – from about 1.6 million barrels per day to about two million now.
Most of Mend’s attacks have targeted pipelines and supply terminals in the south.