ISLAMABAD — About 25,000 Pakistanis angry at a government they say is corrupt and indifferent to the plight of common citizens descended on the capital Monday, responding to the call of a charismatic cleric who has quickly become a powerful but mysterious political force.

The dramatic entry into Pakistani politics of Tahir-ul-Qadri, a preacher who until recently lived in Canada, has sparked concern from some that he is seeking to derail elections expected this spring. Undercutting the elections could set the stage for the country’s powerful army to take power.