The Tehreek-e-insaaf (PTI) political party organized the protest to coincide with the national holiday, declaring it a "march of independence," to seek the removal of Prime Minister Nawaz Sharif.
Sharif has been accused of vote rigging during last year's election.
Around 100,000 people are expected to join Thursday's protest.
On Wednesday, Sharif publicly addressed the accusations. He refuted any wrong-doing and pointed to improvements made since his election with regard to "currency value, foreign investment and economic growth." The prime minister also offered to form an investigative commission comprised of three Supreme Court judges to examine the allegations of electoral fraud.
The prime minister's response to the corruption allegations is too little, too late, opposition leader Imran Khan said at a press conference following Sharif's remarks.
The demonstration, led by Khan and anti-government cleric Tahir-ul-Qadri, was expected to start at 6:00 p.m. local time (9 a.m. EDT) in Islamabad's "red zone," an area of government buildings.
Shortly after the demonstration officially began, Khan posted a photo to Twitter, remarking on the "Amazing crowds on both sides of the Mall," adding "Change will come Insh'allah."