Twenty Polish soldiers held by militants inside an Iraqi police building were to spend the night there, Polish officials said.
The soldiers had gone to help Iraqi policemen in the town of Hilla who had been surrounded by about 250 protesters loyal to Iraqi cleric Moqtada Sadr.
"We don't want to endanger our soldiers by moving them out during the night," Lt Col Artur Domanski said.
The Poles command a 9,000-strong 21-nation force in south-central Iraq.
"The building is specially fortified and the soldiers are well equipped, so I can be certain that they will last until morning," Col Domanski said.
Polish military officials in Warsaw said no shots had been fired.
Elsewhere in Iraq, a temporary truce appears to be in place in the holy city of Najaf, which is reported calm after more than a week of heavy fighting.
Mr Sadr has set out conditions for an end to the clashes between his men and the Iraqi-US force.
The joint US-Iraqi offensive in Najaf has triggered pro-Sadr protests in a number of towns and cities.
Thousands of demonstrators gathered on Friday outside Baghdad's Green Zone, where most of the Iraqi government ministries are housed.