"All over the world today women are speaking up. Their stories are still not really heard," Adichie said at the opening of the world's biggest publishing event.

"Women are still invisible. Women's experiences are still invisible."

A year after the #MeToo movement went viral and ignited a global discussion about sexual harassment, Adichie said there was much work left to be done.

"It is time for us to pay more than lip service to the fact that women's stories are for everyone," the celebrated author of Americanah and Half of a Yellow Sun said.

"We know from studies that women read books by men and women. But men read books by men. It is time for men to read women."

As well as being an award-winning novelist, Adichie shot to fame as a vocal advocate for women's rights after her essay "We Should All Be Feminists" became a TED talk hit in 2012. It has been viewed over five million times on YouTube.

In a thinly veiled reference to US judge Brett Kavanaugh's controversial confirmation to the Supreme Court despite accusations of sexual misconduct, Adichie slammed the tendency not to believe victims of such assaults.

"We seem to live in a world where many people believe large numbers of women can simply wake up one day and make up stories about having been assaulted," Adichie said.

"I know many women who want to be famous. I don't know one single woman who wants to be famous for having been assaulted."

Read also: Uncensored: Frankfurt book fair gets political in 'stormy' times

- 'Call a lie a lie' -

Adichie, who divides her time between the United States and Nigeria, said now was an "urgent" moment to stand up for what is right -- particularly in President Donald Trump's America.

"The world is shifting, it's changing. It's darkening," the 41-year-old said.

"The most powerful country in the world today feels like a feudal court, full of intrigues feeding on mendacity, drowning in its own hubris.

"We must know what is true. We must say what is true. And we must call a lie a lie."

Adichie was due to receive the PEN Pinter prize in London later on Tuesday, which recognizes British and Commonwealth writers whose work takes an "unflinching, unswerving" look at the world.

Judges praised Adichie's "understanding of gender, race, and global inequality".

Adichie's appearance at the Frankfurt fair, which runs until Sunday, kicked off what promises to be a politically charged edition of the annual literature extravaganza, with organizers shining a spotlight on human rights and freedom of expression.