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Ceremonies are to be staged around the world to mark one year since more than 200 girls were abducted by Nigerian militant Islamist group Boko Haram.
A procession will be held in the capital, Abuja, with 219 girls taking part to represent each missing girl.
Similar marches are planned worldwide, including in London and Washington.
On Tuesday, Amnesty International said 2,000 girls and women had been abducted since the start of last year, becoming cooks, sex slaves and fighters.
The abduction of the girls in Chibok in north-eastern Nigeria on 14 April 2014 sparked global outrage, with nations such as the US and China promising to help find them. But to date, none has been traced.
The girls’ plight drew mass attention on Twitter last year under the #BringBackOurGirls hashtag, with the campaign joined by high-profile figures such as Nobel Peace Prize winner Malala Yousafzai and US First Lady Michelle Obama.
Analysis: Will Ross, BBC Nigeria correspondent
It has been a whole year of agony for the relatives of the missing 219 Chibok girls. There have been a few sightings of some of the abducted students but very little official information from a government that has long promised to rescue them from the clutches of Boko Haram.
One mother told the BBC she sometimes arranges her 19-year-old daughter’s clothes in the hope that she is about to return home.
The scale of this conflict is so grim that the Chibok girls represent just a fraction of those seized by the jihadists. Amnesty International says at least 2,000 women and girls have been abducted since the beginning of last year. Many have escaped partly thanks to a recent military offensive – but not the Chibok girls.
Campaigners have criticised the Nigerian government of outgoing President Goodluck Jonathan for not doing enough to find the girls and combat the six-year Boko Haram insurgency in the north.
But Mr Jonathan told the BBC’s Newsday that politics had come before the welfare of the girls.
He said: “There’s so much about these girls that one can’t even comprehend. So much… more of politics than concern about the girls and the stories are twisted and painted different colours.”
Nigeria’s incoming president, Muhammadu Buhari, said his government would “do everything in its power to bring them home”, but added: “We do not know if the Chibok girls can be rescued. Their whereabouts remain unknown. As much as I wish to, I cannot promise that we can find them.”
‘Sexual slavery’
A week of events has been organised by the Bring Back Our Girls campaign group in Nigeria to encourage people to remember the girls ahead of the anniversary.
In an open letter on Monday, Ms Yousafzai called on Nigeria’s authorities and the international community to do more to secure the release of the girls.
Esther, a mother of an abducted girl, told the BBC: “At night I sometimes wake and wonder, and pray to God to deliver the girls… Whenever I want to eat what comes to my mind is, has my daughter eaten?”
While little has been heard of the missing girls since they were taken from their boarding school, one woman told the BBC that she saw more than 50 of them alive three weeks ago in the north-eastern town of Gwoza.
In its report on Tuesday, Amnesty International said the Chibok girls “were only a small proportion” of those abducted.
It based its report on nearly 200 witness accounts, including 28 women and girls who escaped captivity.
One month after the Chibok attack, Boko Haram leader Abubakar Shekau appeared in a video explaining that the girls had been converted to Islam and married off, fuelling concern that the militants had treated them as war booty and sex-slaves.
Mr Buhari has vowed to “crush” the insurgents. He is due to be inaugurated on 29 May after defeating Mr Jonathan in last month’s presidential elections.
BBC