Thousands of Pakistani opposition supporters, some armed with sticks and wire cutters, marched toward a fortified zone in the center of Islamabad on Tuesday to press their demands for the resignation of the prime minister.
The protesters, who have been camped out in the capital since Friday, are led by Imran Khan, the former cricketer, and a charismatic cleric named Muhammad Tahir-ul-Qadri, who run separate campaigns but are united in their opposition to Prime Minister Nawaz Sharif.
The march on the city's "red zone" - an area that contains the Parliament, the prime minister's official residence and many Western embassies - was widely seen as a final effort by Khan to rally his support base after days of threats and political speech.
Although Khan's crusade attracted a crowd in Islamabad that was smaller, it received a major lift on Tuesday when his supporters merged with Qadri's, forming a crowd that the police estimated at more than 40,000 people marching into the red zone.
As participants started toward the Parliament building, television pictures showed a crane on the street that was apparently used to remove shipping containers impeding their way.
Sharif's government, which came to power in June 2013, has struggled to quell the escalating political crisis, partly as a result of Sharif's tense relationship with the Pakistani army leadership.
In recent days, Sharif's administration failed to engage Khan and Qadri in negotiations to end the standoff and appeared to be hoping that the protests would simply fade.
But there was little sign of that Tuesday, as Khan and Qadri gave impassioned speeches before sending their followers toward the city's protected area, which was ringed by shipping containers and thousands of police and paramilitary officers.
In his speech before a cheering crowd, Khan repeatedly attacked Sharif, whom he accuses of stealing the 2013 election through vote rigging, and challenged him to a "duel."
Describing the prime minister as a thief and a corrupt politician, Khan vowed to turn the space outside the Parliament into "a Tahrir Square," a reference to the site of the 2011 uprising in Egypt. While instructing his jubilant supporters to remain peaceful, he repeatedly warned of the possibility of violence.
"Nawaz Sharif," he told the crowd, directly addressing the prime minister. "If there is any violence, I will not leave you." Moments later, when a helicopter hovered nearby, Khan said it had come to "take away Sharif," drawing a roar of approval.
The government said that 30,000 security forces had been deployed to protect the red zone, which included the U.S. Embassy. Witnesses said they could not see evidence of such a large contingent, but the army said in a statement that it had deployed 700 soldiers to protect the Parliament, the Supreme Court and other important buildings inside the zone.
The decision to deploy army troops was taken after a meeting between Sharif and the army chief, Gen. Raheel Sharif, who is not related to the prime minister.
"The army is not behind anyone," said Chaudhry Nisar Ali Khan, the Pakistani interior minister, during a press briefing earlier in the day.
The military urged dialogue after thousands of protesters reached the front of the Parliament late Tuesday night.
A military spokesman said all buildings in the red zone were a "symbol of state and being protected by army, therefore the sanctity of these national symbols must be respected."
The statement came within minutes after Khan demanded that Nawaz Sharif resign by Wednesday evening. "Otherwise, I will storm into the prime minister's house," Khan said.
By midnight, Qadri's supporters seemed to be the first to reach Constitution Avenue, which runs in front of the Parliament. Farther back, Khan exhorted his supporters to move forward from the top of a container.
Aides to Sharif insisted Tuesday that he would not resign. "There is no question the prime minister will resign," a close aide to Sharif said.
Sharif's officials have privately said they feared the opposition marches could provide a pretext for the military to intervene - much as it did in 1999 when his last stint in power came to an abrupt end with a military coup.
Many suspect that Qadri, the cleric, has at least tacit backing from the military. Qadri denies any such links and is openly supported by the leaders of Barelvi and Shiite Islamic organizations, which has helped to galvanize his dedicated supporters.
Whatever the outcome of the protests, analysts said they appeared to have opened a turbulent political period.
In a reminder of the pressing security challenges facing Pakistanis, the military said Tuesday that it had killed 48 people during airstrikes against Taliban camps in the North Waziristan and Khyber tribal districts.