Dozens of people have been killed in the two English-speaking regions since October after a violent crackdown on protests against the mainly French-speaking government.
Many English-speakers have accused the Francophone majority of discrimination and that has fuelled a separatist movement.
Army spokesperson Colonel Didier Badjeck said the curfew would come into force on Saturday.
It will last from 20:00 to 06:00 and is due to concerns of an "imminent attack" in "several cities" by separatists, backed by "mercenaries" from neighbouring Nigeria, a defence ministry memo said.
The curfew can be renewed, said a note from the governorate of the English-speaking north-west region, seen by AFP.
Cameroon's southwest and northwest regions are home to an English-speaking minority that accounts for about a fifth of the population.
In October, separatists declared the two regions as the self-proclaimed republic of "Ambazonia", prompting a forceful reaction by the government.
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Police say 22 police and soldiers have been killed in the violence but officials in the capital Yaounde have not released figures for the number of separatist or civilian losses.
The spiralling violence has prompted an estimated 30 000 Cameroonians to flee to neighbouring Nigeria.
Biya, 84, who has held power since 1982, on Saturday called on young Cameroonians to be "patriotic internet users" and said the situation in the anglophone areas had improved after "troubles which sometimes sparked acts of violence."
It comes after a string of grisly video clips circulated online alleging atrocities against separatists by Cameroonian soldiers.
On Friday, the Cameroonian army denied its troops were responsible for the alleged gruesome killings in English-speaking regions in the unverified video.
"These accusations of atrocities are dreamt up. We are victims of disinformation and fake news," said army spokesperson Colonel Didier Badjeck.