A Cairo-based rights group says a prominent activist imprisoned in Egypt has joined others in an ongoing hunger strike to protest the government's crackdown on dissent.

The Arabic Network for Human Rights Information issued a statement Monday saying at least a dozen protesters and activists behind bars have gone on hunger strike in the past week. The group says the rise is in reaction to "unfair trials" and "noticeably prolonged detentions" without trial in cases involving freedom of expression.

The group said Mahienour el-Masry, a secular activist sentenced for violating a controversial protest law, started a hunger strike Sunday.

Security officials say at least 20,000 people have been detained since the government crackdown began last year. Most of the detained are Islamist supporters of Egypt's ousted President Mohammed Morsi.