Israeli right-wingers who hunted Arabs through the streets of Jerusalem to seek revenge for the killings of three murdered teens have reportedly gone around the city asking dark-skinned people "what's the time?" in Hebrew to check for an Arab accent.

Grim details have emerged about Monday's protest, with footage showing Israeli ultra-Orthodox youth and right-wing activists chanting slogans such as "death to Arabs" "a Jew is a brother, an Arab is a bastard," "we want war" and "Mohammed is dead".

Haaretz reported that five Palestinians were attacked by extremists who engaged in clashes with police in Jerusalem's city centre. Two Palestinians needed medical treatment, and police arrested 50 people suspected of involvement in the incidents.

About 400 demonstrators blocked the road under Chords Bridge, by the city's western entrance.

The main purpose of ultra-Orthodox viglantes was to reach the Old City through Jaffa Gate to attack Arabs. Some of them stormed a McDonald's because they heard that Arabs were employed there and police had to force them out.

A group of a hundred protesters attacked three Palestinian workers in a downtown parking lot. The workers were having their evening meal at the end of the daily Ramadan fast. One of the workers suffered a head injury after the mob assailed him.

"I was in the middle of a Ramadan meal. Someone passed by and noticed I am an Arab," Mohammed Sayyad of A-Tor told Haaretz. "He called out to everyone else, saying 'Here's an Arab, here's an Arab.' They started throwing stones and spraying gas, and cursing, 'Tomorrow we're going to kill you.'"

Meanwhile, the family of Naftali Frenkel, one of the murdered Israeli teenagers, has condemned the apparent revenge attack on a Palestinian teenager.

"If the Arab youth was murdered because of nationalistic motives then this is a horrible and horrendous act. There is no difference between [Arab] blood and [Jewish] blood. Murder is murder. There is no forgiveness or justification for any murder."

The Palestinian teen, named Mohammed Abu Khdair, from annexed east Jerusalem was kidnapped and killed in an apparent revenge attack.

The youth was forced into a car in Beit Hanina, according to Israeli army radio. Police received a call about a suspected kidnapping and started a widespread search operation.

"Within an hour, a body was found in Jerusalem that has still not been identified. We are looking to see if there is a connection between the two incidents," police spokeswoman Luba Samri told AFP.

The body, charred and with signs of stab wounds, was found in a forest near Givat Shaul in west Jerusalem.

Israeli PM Benjamin Netanyahu has ordered a swift investigation to solve Palestinian youth's "despicable" murder and called on both sides not to take the law into their own hands.

"Israel is a state of law and everyone is obligated to act in accordance with the law," he said.