The demonstrators also burned posters of both Trump and Bahrain's monarch King Hamad bin Isa Al Khalifah.
Mahmoud Barhoush, 25, said he didn't know whether to laugh or cry at what he called Arab states' "treasonous" participation in the US-led confab.
“Enough of your running into the arms of Trump and [Israeli Prime Minister Benjamin] Netanyahu,” he said at the Ramallah protest.
In the Gaza Strip, crowds burned large paintings of Trump marked with the words: “Deal of the Devil.”
“Down with Bahrain, down with Trump, down with the Manama conference,” chanted the demonstrators.
“The Manama conference is a comedy show, a wedding without the bride (the Palestinians) ... it will not succeed,” said a protester who gave her name as Siham in Gaza City.
Also on Monday, Palestinian leaders renewed their calls for a boycott of the Manama conference.
“This workshop is simply a political laundry for settlements and a legitimization of occupation,” Palestinian Prime Minister Mohammad Shtayyeh told journalists ahead of a cabinet meeting.
Hamas spokesman Hazem Qassem also said, “They (Palestinians) will not sell out their rights for all treasures on earth.”