The protest by Polish transport companies is the latest in a series of economic clashes between the neighbours, who are traditionally close allies but have seen relations suffer in recent months.
"Traffic on the Ukrainian-Polish border has remained complicated (for lorries) since November 6 in three directions," Andriy Demchenko, Ukraine's border service spokesman said.
Kyiv estimated that around 1,700 trucks were in queues waiting to enter Ukraine.
Poland's border service had published on its website estimates of the wait time at entry points into Ukraine.
It estimated that crossing by freight lorry at the Hrebenne checkpoint would take "150 hours", while the wait at the border at Medyka, further south, would take "55 hours".
The Polish protesters want entry permits to be reinstated for their Ukrainian rivals.
The European Union waived permits for Ukrainian transport companies entering the 27-nation bloc, of which Poland is a member after Russia invaded Ukraine last year.
Kyiv has said the Polish truckers' blockade harms the interests of both countries and that it has not received any official demands from Polish transport companies.
"It is wrong to close the border with Ukraine during the war," Oleksandr, a 36-year-old trucker who had been queueing for two days at the Rava Ruska crossing, told AFP. He declined to give his surname.
Warsaw has been Kyiv's staunchest backer against Russia since Moscow's invasion in February 2022.
But relations between the neighbours took a downward turn during Poland's general election this autumn when its governing nationalist party became embroiled in several rows with Kyiv.