LAHORE, Pakistan – Thousands of opposition protesters on Thursday joined large convoys headed to Pakistan’s capital Islamabad for a mass rally to demand the ouster of the prime minister over allegations of vote fraud.

The rally is seen as the strongest challenge yet to the government of Prime Minister Nawaz Sharif, just a year after he took office in the first democratic transfer of power in a country long plagued by military coups.

The protesters set out on Thursday morning from the city of Lahore in cars, trucks and buses, while others walked or drove motorcycles as they embarked on the 300 kilometre (187 mile) journey to Islamabad. Initially 5,000 protesters were on the march, but the number steadily increased. The slow-moving convoy covered just five kilometres in seven hours.

The convoy is led by Imran Khan, a famous cricketer-turned-politician who heads the Tehrik-e-Insaf party, the third-largest in parliament. The demonstration was called to coincide with Pakistan’s Independence Day.

“I am going to Islamabad to seek resignation from Nawaz Sharif,” Khan told supporters in a speech Thursday evening.

“Get ready to win the match,” said Khan, who is revered in Pakistan for leading the national team that won the 1992 cricket World Cup.

Sharif’s ruling party has rejected Imran’s demand, saying the elected democratic government would complete its term, which ends in 2018.

Thousands of policemen were deployed across Islamabad and along the convoy’s route while the capital’s entry points have been blocked since earlier this week with large shipping containers. Interior Minister Chaudhry Nisar Ali Khan told reporters that the government had allowed peaceful rallies but that “anyone trying to disrupt peace would be dealt with an iron hand.”

He said authorities had made arrangements to handle as many as one million people in Islamabad.

The protesters were moving slowly and were expected to arrive in Islamabad on Friday, a day later than planned.

Khan’s convoy got off to a colorful start in Lahore, with protesters dancing to the beat of drums and singing patriotic songs. Many women had the green and white of the Pakistani national flag painted on their cheeks, along with the red and green of Khan’s party.

A separate march Thursday was led by Tahir-ul-Qadri, a Pakistani cleric who is also a Canadian national and who commands a loyal following of thousands through his network of mosques and religious schools in Pakistan. He left Lahore with thousands of his followers and was expected to join Khan’s rally on the road or in Islamabad.

Qadri’s convoy also moved slowly, covering a distance of eight kilometres in seven hours.

“Over 200,000 people are with me and I am going to Islamabad to bring a peaceful green revolution in Pakistan,” he told The Associated Press by phone from Lahore. Qadri refused to hold any talks with Sharif and urged him to step down.

Both Qadri and Khan have called for new elections, claiming last year’s vote was invalid due to widespread rigging by government supporters.

“We are out on the streets to do our struggle for a change in the system,” said one of the protesters, Mohammad Faheem.

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.

Sharif, who was overthrown in the 1999 coup that brought former army chief Pervez Musharraf to power, has met regularly with top advisers ahead of the rally. The government has also invoked a rarely-used article in the constitution allowing the military to step in to maintain law and order if needed.

Speaking at an Independence Day ceremony in southwestern Pakistan, Sharif criticized the rally, calling it “negative politics.”

Sharif said Khan would be better advised to “work to alleviate poverty and improve law and order” in Pakistan.

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Associated Press writers Asif Shahzad, Zarar Khan and Ahmed in Islamabad contributed to this report.