The group's advocacy coordinator, Rafat Sub Laban, told Ma'an on Monday that at least 988 Palestinians had been detained in October, including 578 in the occupied West Bank, 258 in East Jerusalem, and 152 in Israel.
He added that it was not yet clear how many prisoners were still being held and how many had been released, although Addameer estimated there were some 6,300 Palestinians in Israel's prisons in total, up from 5,620 at the end of last month.
Sub Laban added around 400 prisoners are currently being held under administrative detention -- internment without trial or charge -- including 43 Palestinians detained this month.
He also said that there are now some 270 children, under the age of 18, inside Israel's prisons, as well as 36 women.
The activist noted that although the numbers were as accurate as possible, they remained approximate, based on the group's own documentation.
"We're not getting accurate information from inside the jails," he said.
Other monitoring groups have said that as many as 1,200 or 1,300 Palestinians have so far been detained in October.
On Sunday, the Palestinian Government Media Center said that 1,003 Palestinians had been detained, which it said put the number of Palestinians in Israel's prisons as high as 6,620, with around 500 under administrative detention.