The arrests come after President Mahmud Abbas made a pledge to use an "iron fist" to enforce a truce, security sources said on Friday.
They were the first arrests by the Palestinian authorities over ceasefire violations since Abbas won agreement from armed groups to respect the truce at a meeting in March.
Israel has complained Abbas is not doing enough to rein in fighters.
A Palestinian security source said two members of the al-Aqsa Martyrs Brigades, part of Abbas's ruling Fatah movement, were arrested by the preventive security force on Thursday.
They were being questioned about rockets fired at Gaza settlements that Israel plans to evacuate in coming months.
A spokesman for the brigades said: "We urge the Palestinian Authority and Abbas to release our two fighters."
Abbas, in his toughest pledge yet to enforce the ceasefire, told the police two days ago that they should use force to prevent fighters from flouting the truce, which has raised hopes for peacemaking after four and a half years of a Palestinian uprising.
"We urge the Palestinian Authority and Abbas to release our two fighters"
Al-Aqsa Martyrs Brigade spokesman
Occasional mortar, rocket and shooting attacks have continued even though major Palestinian factions, including the al-Aqsa Martyrs Brigades, say they are committed to calm. Some groups say they are not bound by the deal.
The United States welcomed Abbas's pledge to enforce the truce.
But reflecting differences between Israel and Washington, Israel's Haaretz newspaper said the Jewish state had balked at a US request that Palestinian police be allowed to be armed.
US officials declined to comment.
Israel is also uneasy over an expected offer from Russian President Vladimir Putin, visiting Abbas in the West Bank, to supply armoured vehicles to Palestinian security forces.
"We want to see security forces taking action. We require them to disarm fugitives and put them under probation. What purpose will weapons serve if they are not taking any action?" a senior Israeli official said.
Abbas is adamant he wants to
maintain a truce with Israel
Abbas forced hundreds of security men into retirement last week including long-time loyalists to the late Yasir Arafat, whose replacement by Abbas has improved relations with Israel.
Israel, which wants to ensure calm during its withdrawal of settlers from occupied Gaza, has said there will be no full peace negotiations until Palestinians dismantle armed groups.