Imran Khan, a famous cricketer who now leads the country's third-biggest political bloc, and a fiery anti-government cleric called for the rallies in Islamabad, focused on making Prime Minister Nawaz Sharif leave office and holding new elections.
Sharif, who took office just a year ago in the first democratic transfer of power in a country long plagued by military coups, has said he'll stay in power, raising fears of possible political instability in this nuclear power.
`'I'll sit here and Nawaz Sharif, you decide. You have just one option - resign and hold re-elections,'' Khan told supporters gathered Saturday in the lashing rain.
Khan said that the current leadership was unacceptable. ''We do not recognize them, we have to get justice, we have to get freedom from these types of rulers,'' he said.
Rana Sanaullah, a senior leader of Sharif's ruling party, said the government was ready to investigate allegations of electoral fraud, but ruled out Sharif could be removed through a rally. Sharif won a landslide election in May 2013.
The protesters left the eastern city of Lahore on Thursday, vowing to march to the capital and camp out there until their demands for a new government are met. Despite the darkness and the lashing rain, the crowds swelled as they entered Islamabad shortly before late Friday night.
An anti-Taliban cleric Tahir-ul-Qadri also reached Islamabad late Friday. He too led tens of thousands of his supporters to bring about what he called a ''green revolution.'' A spokesman for Qadri, Shahid Mursaleen, said the cleric would also deliver a speech Saturday to call for Sharif's removal and immediate arrest.
Security has been tightened across the capital amid fears of unrest in a country with a long history of chaotic politics and military coups. Authorities placed shipping containers into the road to block traffic in many areas, disrupting normal life in the city.
Police estimate some 60,000 people were taking part in the rallies.
Khan late Friday said a police officer gave him a letter from the provincial Punjab government, warning that the Punjabi Taliban had planned to kill him.
''I thought, if my life has to come to an end, better it goes in struggle to get real independence for the nation,'' he said.
On Friday, as the march led by Khan passed through the city of Gujranwala, supporters of Sharif's Pakistan Muslim League-N hurled stones at the convoy, said Khan, who was unharmed.
Mohammed Azeem, a police officer in Gujranwala, about 40 miles (70 kilometers) from Lahore, said some 200 ruling party supporters clashed with Khan's protesters but that ''the situation is under control.''
Both Khan and Qadri have vowed to bring 1 million followers into the streets of Islamabad, a city of roughly 1.7 million inhabitants.
Pakistan, a nuclear-armed country of 180 million people, has largely been ruled by military dictators since it was carved out of India in 1947. The army still wields great influence in Pakistan, which is battling several militant groups, but has not taken sides in the protests. There are fears, however, that political unrest could prompt the military to intervene.