Jamayyal was "one of our own", one mourner said, and his funeral was a bitter mark on the road to increasing anger at the Palestinian Authority (PA), welcomed only seven months ago.
Jamayyal, 27, was declared brain dead on Monday after being taken to hospital from prison in Nablus. His lawyer said he had serious burns, lacerations and had been savagely beaten. The hospital said he had suffered massive internal bleeding and was kept alive by respirator until yesterday.
Haj Ismail, commander of the PA security forces in the West Bank, went to Jamayyal's family home to return his body. Jamayyal's father refused to accept it until his other son, Muayyed, who has been held for seven months without charge, was released. As a huge crowd gathered, the police relented. Muayyed was released and the family moved to bury Mahmoud.
Thousands followed the procession to the cemetery, some chanting the names of the nine other young men who have been killed under interrogation by the Palestinian police since they were first deployed in Gaza and parts of the West Bank two years ago. Police agreed to stay away.
Nablus observed a commercial strike yesterday. The city was a centre of resistance to Israeli occupation and has had a stormy relationship with the PA since it took over last December. Hostility reached a peak in March when police raided the An-Najah university and arrested more than 100 students.
Those arrests were part of a crackdown on Islamist militants following a wave of suicide bombings in Israel. While Israel and America demanded the PA crush Hamas, many who wound up in jail were merely observant Muslims, caught in the wrong place at the wrong time by a police force desperate to show it was doing something.
At least 1,000 people are still being held in Palestinian prisons, without charge. Thirty nine prisoners in Juneid prison in Nablus are on the 12th day of a hunger strike. The Palestinian Human Rights Information Centre says it has "dozens and dozens" of documented reports of torture.
Jamayyal, however, was not a Hamas supporter. He was a member of the Fatah Hawks, the militant branch of Yasser Arafat's dominant political faction, and was a popular leader during the Intifada rebellion.
Days after taking over Nablus, the Palestinian police went after the Fatah Hawks. Jamayyal was detained less than a month after the police arrived. His family only learned of his whereabouts when a nurse recognised her patient.
The rising anger in the West Bank and Gaza is matched only by increasing fear. "The PA took the free hand it was given by Israel and America to crack down on Hamas and they did that, and they used all the torture skills the Israelis taught them . . . now they're turning everything they learned against their own people, determined to wipe out every shred of opposition," said one human rights activist.