Israeli police have arrested Palestinian presidential candidate Mustafa Barghouti, his aides say.
Barghouti has sometimes found campaigning an uphill struggle
Mr Barghouti was taken away to the Central Police Station as he was campaigning in East Jerusalem's Old City area, witnesses said.
The arrest comes a day after Israel adopted measures said to be aimed at facilitating Palestinian elections due to take place on 9 January.
These included allowing candidates to campaign within East Jerusalem.
In a separate development, Israel released about 160 Palestinian prisoners as an act of goodwill towards Egyptian President Hosni Mubarak, but Palestinians dismissed the move as a public relations stunt.
Most were low-level security detainees approaching the end of their sentences.
They were released at four checkpoints in the West Bank and at the Erez crossing into the Gaza Strip, where there were emotional family reunions.
Also, a wanted local leader of the militant al-Aqsa Martyrs Brigades was shot dead by Israeli special forces dressed in civilian clothes in Nablus in the northern West Bank.
Witnesses said Wael Riyahi, 35, was trying to evade arrest. The Israeli army said he was killed after trying to run over soldiers attempting to arrest him
Mr Barghouti was "detained for questioning because he has the right to transit through Jerusalem but not be in Jerusalem itself", a Jerusalem police spokesman said.
It is the second time he has been detained by Israeli security forces during the campaign.
Today's prisoner releases failed to impress Palestinians
East Jerusalem was captured by Israeli forces in 1967. It is considered occupied territory under international law, but Israel claims the whole city as its exclusive capital.
Opinion polls suggest Mr Barghouti, a human rights activist, is running second to interim Palestinian leader Mahmoud Abbas in the race to succeed Yasser Arafat, who died on 11 November.
Earlier this month, Mr Barghouti said Israeli troops struck him and his aides as they tried to cross an Israeli military checkpoint in the West Bank.
Israel said his team had refused to allow a vehicle search and were briefly detained.
Mr Barghouti has complained that while Israeli forces have allowed free passage to Mr Abbas - thought to be Israel's preferred successor - he and four other candidates have faced numerous obstructions as they try to campaign.