Laura Kwoczała, 24, was travelling as part of the Global Sumud Land Convoy – a humanitarian initiative aimed at breaking Israel's blockade of Gaza – when she was stopped at a checkpoint on Sunday by forces loyal to General Khalifa Haftar, the self-proclaimed ruler of eastern Libya.
Organisers say they lost contact with Kwoczała and nine other convoy participants at around 4pm local time.
Her condition is unknown.
Italian media report that the detainees, including two Italian nationals, were taken to Benghazi, where they are being treated as irregular migrants.
They are accused of entering the region without a permit required by Haftar's authorities, and face possible deportation while awaiting trial.
Foreign ministry spokesman Maciej Wewiór confirmed consular staff were in contact with local Libyan services, but gave no further details.
The land convoy set out from Algeria and is part of a broader international effort that also includes the Global Sumud Flotilla, which sailed from Turkey in mid-May.
Spokesman Rafał Piotrowski said the convoy comprises more than 200 participants from 25 countries, and that Kwoczała's group joined it in Libya on 7 May before being stopped on 17 May, some six kilometres from the Libyan eastern border.
Israel has described the Sumud initiative as backed by Hamas, which it designates a terrorist organisation.
Libya has been divided since the killing of Muammar Gaddafi in 2011, with an internationally recognised government controlling the west and Haftar's forces holding the east.
Poland's foreign ministry advises against all travel to Libya and urges any Polish nationals still in the country to leave immediately.
(ał)
Source: PAP, IAR