Supporters of Tahir-ul-Qadri are expected to arrive in Islamabad on Monday.
Thousands of police have been deployed in the capital, and police in riot gear are manning the main roads and streets of Islamabad.
Authorities have used shipping containers to block off the part of the city where most government offices and embassies are located.
Qadri returned to Pakistan in December after living for years in Canada.
His calls for vague reforms ahead of elections this spring have galvanized many Pakistanis disenchanted with the political system.
But he's worried observers who fear the cleric is a front for the military to derail the democratic process.Suggest a correction