On 1 August 1944, the Polish Home Army (the Armia Krajowa, or AK) launched a major operation in an attempt to liberate Warsaw from Nazi German control. This operation would come to be known as the
Warsaw Uprising.
Initially, the AK was able to gain control over large portions of the city. However, the Russians chose to withhold any assistance, and over the next two months, the Germans eventually gained the upper hand. On 2 October 1944, the final AK elements surrendered, returning full control of the city back over to the Germans.
During the 2 months in which the Poles controlled portions of the city, there were several provisional postage stamps designed and/or issued for use. Here are a few.
The first overprint issue consisted of overprints of the Generalgouvernement Hitler Head issues. The overprint reads "Poczta Polowa 1-VIII-1944 Warszawa", or "Fieldpost 1 August 1944 Warsaw". These were never issued for use.

Fischer I-X

Fischer I-X on album page
The second set consisted of overprints of common Generalgouvernement stamps with a design featuring an eagle and the text "Poczta powsta#324;cza w VI sierpie#324; 1944", or "Post of the Uprising of 6 August 1944".

Fischer 1-112 overprint
In addition, the AK had Fieldpost issues. This set was issued on 3 September 1944. There were five colors, one for each of the districts of Warsaw held by the AK. The text reads "Poczta polowa", or "Fieldpost", with "AK" for the Armia Krajowa, and "1. VII 1944", the date of the uprising.

Fischer 113, 114, 115, & 117 on piece

Fischer 114-116
Finally, a second AK Fieldpost issue was designed, but it is not believed to have been used due to the end of the uprising.

Fischer 118

Fischer 114-116, 118 on album page
For more info on the Warsaw Uprising, see
Wikipedia. For more on the stamps of the Warsaw Uprising, see the
Stamp Encyclopaedia Poland.