JERUSALEM — A prominent critic of the Palestinian Authority in the West Bank died in the custody of Palestinian security forces soon after being arrested at his house Thursday, according to Palestinian officials and the man's lawyer.

His family said that officers who stormed his bedroom began beating Nizar Banat while he was still in bed and that he was bleeding heavily from head wounds as they removed him from the house.

Banat was a well-known anti-corruption activist and opposition candidate who frequently called for Palestinian leaders, including President Mahmoud Abbas, to be held accountable for government failures and malfeasance.

According to his lawyer, security officers entered the house in Hebron where Banat was staying about 3:30 a.m. Thursday, saying they had a warrant for his arrest. An unidentified person posted on Banat’s Facebook page within minutes that officers were taking him. “They have confiscated his computer, phone and all his belongings, and are brutally beating him,” the post read.

Palestinian officials in Hebron announced his death in statement shortly before 6 a.m.

“Following an arrest summons for Nizar Khalil Muhammad Banat at dawn today, security forces arrested him, during which his health deteriorated, and he was immediately transferred to Hebron government hospital for examination, where it was found that he had passed away,” the statement said.

Banat’s family said in interviews with Arab media that he had been in good health. They said officers beat him with a metal bar and pepper sprayed him. They called his death a “premeditated assassination.”

Protests immediately broke out in several West Bank cities, including Hebron and Ramallah, where several hundred demonstrators shouted “shame” and “executioners,” naming Abbas and other top Palestinian leaders.

“This is an assassination in every sense of the word,” Banat’s lawyer, Muhannad Karajah, said in an interview. “What happened with Nizar establishes a new situation of political oppression and the threat of assassination for every human rights defender and every political activist.”

Abbas’s office and other Palestinian officials did not immediately respond to requests for comment.

The Biden administration said it was “deeply disturbed” by Banat’s death and called for a “thorough and transparent” investigation.

“We have serious concerns about Palestinian Authority restrictions on the exercise of freedom of expression by Palestinians and harassment of civil society activists and organizations,” State Department spokesman Ned Price said in a statement.

Hamas, the militant group that governs the Gaza Strip and is a political rival of Abbas’s Fatah party in the West Bank, condemned Banat’s killing and held Abbas and Prime Minister Mohammad Shtayyeh responsible.

Karajah said that Banat had been threatened with arrest before by Palestinian officials but that he was motivated to keep up his criticism, in campaigns and on social media, of a government that he saw as corrupt and ineffective. Banat, 42, had been subjected to threats and an attack by gunmen with stun grenades on his house in Dura about six weeks ago, Karajah said. That assault had led Banat to sleep at his uncle’s house in Hebron.

The attack in Dura came soon after Banat, who had planned to run for office with an opposition party, lambasted Abbas in April when the president, who was lagging in polls, abruptly canceled the Palestinians’ first scheduled elections in more than a decade.

Abbas blamed Israel, citing uncertainty about Palestinians living in East Jerusalem being able to participate in the election.

Banat was among activists who called on European officials to cut off financial aid to the Palestinian Authority after the cancellation of the election. That call reportedly infuriated Palestinian leaders. Banat also posted sharp criticisms of Abbas and Shtayyeh last week over an abortive plan for the Palestinian Authority to accept 1 million doses of nearly expired coronavirus vaccine from Israel.

The European Union’s delegation to the Palestinians tweeted that a “full, independent and transparent investigation should be conducted immediately” into Banat’s death. Local human rights activists also condemned Banat’s arrest and death.

“What happened is a heinous crime, and everyone who participated and who gave orders must be arrested and held accountable,” said Farid al-Atrash of the Independent Commission for Human Rights in Ramallah.

“They know that they can get away with it,” Nadia Harhash, a Palestinian activist said of Banat’s alleged killing in custody. “They know that in a few days another scandal will come up and they will go on.”

Taha reported from Ramallah. Shira Rubin in Tel Aviv contributed to this report.