Sanaa Seif, the jailed daughter of prominent human rights advocate and lawyer Ahmed Seif El-Islam, who died on Wednesday, has joined an ongoing hunger strike with several other prisoners and activists in Egypt to protest their ongoing detention, her sister has said.

Seif, also the sister of leading activist and blogger Alaa Abdel-Fattah, who is also behind bars, was arrested over two months ago for her role in a demonstration against a controversial protest law that bans all but police sanctioned protests.

She began an open-ended hunger strike on Thursday to protest against the notorious law and the "unjust" detainment of thousands, her sister, Mona Seif - a prominent rights activist who co-founded an Egyptian movement against military trials of civilians - posted on Facebook on Friday.

Their brother Abdel-Fattah, an icon of the 2011 uprising that toppled long-time autocrat Hosni Mubarak, was sentenced in June to 15 years in jail for breaching the widely criticised protest law. He is now facing a retrial.

The imprisoned duo were granted permits to attend their father's funeral on Thursday, accompanied by police, and were earlier allowed to visit their comatose father in hospital the week before he died.

Abdel-Fattah, an oustpoken blogger who has been in and out of prison since the 2011 revolt, began a hunger strike on 18 August to protest his detainment.

At least a dozen prisoners and activists behind bars have joined the hunger strike in the past week, according to the Cairo-based rights group the Arabic Network for Human Rights Information (ANHRI).

The detainees, according to the rights group, include 11 supporters deposed president Mohamed Morsi who have been held for months for illegal protesting but have not yet been formally charged.

Since the army's ouster of Morsi in July 2013, authorities have mounted a harsh crackdown on Islamists in which thousands have been jailed and hundreds killed or sentenced to death in hurried mass trials.

The campaign has also extended to several youth activists after the protest law was passed late last year, heightening fears of the future of political dissent in Egypt.

Three other symbols of the 2011 popular revolt have also announced they will join the strike: Ahmed Maher, founder of the

April 6 Youth Movement, Mohamed Adel, a prominent April 6 member, and long-time activist Ahmed Douma.

The trio plans to gradually join the hunger strike if authorities do not meet the other detainees' demands, April 6 media advisor Baker Sharkawy told Ahram Online.

Douma is the least likely to join the hunger strike, due to his health, Sharkawy said.

Another socialist activist and rights lawyer jailed for three months for illegal protesting, Mahinour El-Masry, joined the strike on Sunday - but called it off shortly afterwards when elderly prisoners joined her in solidarity.

In June, authorities released a journalist from the Qatar-based Al Jazeera Arabic channel on medical grounds after he had been on hunger strike for over four months to protest his detention without charges.

A Facebook page which tracks the number of prisoners on hunger strikes - called We are Fed Up - says that among those on strike are photojournalist Ahmed Gamal-Zeyada and two other supporters of Morsi, Ibrahim El-Yamani and Mohamed Sultan, who have been on hunger strike for almost seven months while in prison.