Police raided a youth convention for opposition leader Imran Khan's PakistanTehreek Insaaf (PTI) party, charging activists with batons and arresting dozens of people.
Khan, a former Pakistan cricket star, is due to lead the protest on November 2 to demand Prime Minister Nawaz Sharif step down over recent family financial revelations.
PTI leaders were infuriated by the police raid, further fraying relations ahead of the planned protest.
"The government has proved that there is no democracy in Pakistan, it is a monarchy," PTI lawmaker Asad Umar told reporters from the scene.
Nazish Altaf, a PTI youth leader, added: "We will not be deterred by arrests and more people will join us on November 2."
There will be a heavy police presence in Islamabad to deter protesters, with confrontation now appearing inevitable.
Sharif is under growing pressure from opposition parties over his children's offshore bank accounts that were revealed in the Panama Papers leak.
"This will be a decisive and historic gathering, people will come together with resolve to hold the prime minister accountable," Khan told reporters ahead of the raid.
But a notification Thursday banned gatherings in the capital for two months because protests were "likely to threaten public peace and tranquility, cause public annoyance or injury, endanger human life and safety".
