RAMALLAH, PALESTINIAN TERRITORIES (AFP) - Israeli security forces detained Jerusalem's top Islamic cleric as crowds gathered in the Old City following an attack Friday near a highly sensitive holy site that killed two police officers, the cleric's son said.

The grand mufti of Jerusalem Muhammad Ahmad Hussein, the city's highest Islamic authority, had earlier gathered in the Old City with others and condemned the closure of the Al-Aqsa mosque for prayers after the attack. Israeli police said they had no comment.

Hussein's son Jihad Hussein told AFP his father had been taken to a police station near the Old City.

"Until now, we don't know what is going on with my father," he said. One of the mufti's bodyguards, Khaled Hamo, said police "entered the crowd and took the mufti."

The incident came after three Arab Israelis opened fire on Israeli police, killing two before fleeing to the ultra-sensitive holy site where they were also shot dead in one of the most serious incidents in the city in recent years.

Security forces locked down the area and the Al-Aqsa mosque was closed to Friday prayers after the attack in a highly unusual move. Hussein had earlier condemned the closure of the mosque compound for prayers.

"I have very little information about it, but it doesn't mean you should close the mosque for prayers," he told journalists at the Lions Gate entrance to the Old City, near the holy site.

Hussein was detained at the Lions Gate, the bodyguard said.

The three assailants were killed at the site known to Muslims as the Haram al-Sharif and to Jews as the Temple Mount, the location of regular clashes between Palestinians and Israeli police, but gunfire rarely occurs there.

The site includes the Al-Aqsa mosque and the Dome of the Rock.

A number of attacks have occurred around Jerusalem's Old City in recent months, but they have often involved knives.

Video being shared on social media appeared to show gunshots ringing out at the holy site.

"I was standing here and then I heard the shooting. I thought it was fireworks," Basem Badawi, a 60-year-old water seller in the Old City, told AFP.

"But then I saw the police coming from everywhere."

FLASHPOINT SITE

The Haram al-Sharif/Temple Mount is considered the third-holiest site in Islam and the most sacred in Judaism.

It is central to the Israeli-Palestinian conflict, with Palestinians fearing Israel may one day seek to assert further control over it.

It is located in east Jerusalem, occupied by Israel in the 1967 Six Day War and later annexed in a move never recognised by the international community.

Jews are allowed to visit, but not pray there to avoid provoking tensions. The site is administered by the Islamic Waqf organisation.

Waqf officials said its guards at the site had been detained by Israeli police following the attack.

A wave of unrest that broke out in October 2015 has claimed the lives of at least 277 Palestinians, 42 Israelis, two Americans, two Jordanians, an Eritrean, a Sudanese and a Briton, according to an AFP toll.

Israeli authorities say most of the Palestinians killed were carrying out knife, gun or car-ramming attacks.

Others were shot dead in protests and clashes, while some were killed in Israeli air strikes on the Gaza Strip.

The violence has greatly subsided in recent months.