UPDATE 1-U.N. rights office urges Egypt to free blogger, lawyer, journalist

(Adds new arrest figures, para 3, EU concerns, 10-11)

GENEVA, Oct 18 (Reuters) - The U.N. human rights office called on Egypt on Friday to free a prominent blogger, lawyer and journalist allegedly mistreated in custody who are among several thousand people detained since street protests began a month ago.

Officials at Egypt's interior ministry were not immediately available for comment. The state prosecutor's office said in late September that it had questioned up to 1,000 suspects who took part in the demonstrations.

About 3,400 people have been arrested since protests began, including about 300 who have since been released, according to the Egyptian Commission for Rights and Freedoms, an independent body.

"Unfortunately such arrests are continuing, and have included a number of well-known and respected civil society figures," U.N. human rights spokeswoman Ravina Shamdasani told a news briefing.

Protests against President Abdel Fattah al-Sisi in Cairo and other cities have followed online calls for demonstrations against alleged government corruption.

Sisi, first elected in 2014 after, while army chief, leading the 2013 overthrow of Islamist President Mohamed Mursi, has overseen a crackdown on dissent ranging from liberal to Islamist groups - the most severe in recent memory, rights groups say.

Journalist and activist Esraa Abdelfattah was arrested by plainclothes security officers in Cairo on Oct. 12 and was reportedly beaten after she refused to unlock her mobile phone, Shamdasani said. Abdelfattah is on a hunger strike, she added.

Alaa Abdel Fattah, a blogger and software engineer, was released in March after serving a five-year sentence for protesting without permission, but was re-arrested on Sept. 29, Shamdasani said. The same day, his lawyer Mohamed al-Baqer, was arrested while attending the interrogation, Shamdasani added.

Abdel Fattah was struck by guards on his back and neck while being forced to walk down a corridor in his underwear, while al-Baqer has been subjected to physical and verbal abuse, and denied water and medical aid, she said.

Ivan Surkos, the European Union's ambassador to Egypt, tweeted on Tuesday that he had raised concerns over individual detained activists with Ahmed Gamal Eddin, Assistant Foreign Minister for Human Rights.

"Will my messages end as (a) dialogue of deaf persons? Let's hope not," Surkos said. (Reporting and writing by Stephanie Nebehay with additional reporting by Amina Ismail in Cairo Editing by Mark Heinrich)