Driving rain, howling winds and a lightning storm wrecked a meticulously planned show of political strength on Saturday by Imran Khan, the former cricket star who hopes to be next prime minister of Pakistan.

With historic elections set for 11 May Khan's party, the Pakistan Tehreek-e-Insaf (PTI), had been working for weeks to gather a vast crowd of supporters in Lahore, the capital of the Punjab, the crucial province where the bulk of Pakistanis live.

But the heavens opened on an estimated crowd of more than 100,000 people just after Khan finally rose to deliver a speech decrying Pakistan's many problems and promising to sweep away the country's established political parties.

The downpour sparked jokes on social media about the unplanned "tsunami", the word Khan frequently uses to describe a movement made up largely of frustrated young first-time voters desperate for change in a country facing economic collapse, crippling energy shortages and domestic terrorism. Khan had barely got through a couple of his promises to the nation before both the crowds and the VIPs surrounding him on a raised stage broke for cover.

Enthusiastic crowds, largely made up of the army of young people who have become the shock troops of Khan's PTI, had poured into a park in the shadow of Lahore's historic Badshahi mosque. "So many people are here because we are sick of the main parties," said Nadeem Shahzad, a bus driver. "They are money-making machines, only interested in making themselves rich."

The gathering in a sprawling park surrounding the Minar-e-Pakistan, a monumental tower built to commemorate the 1940 demand for an independent Muslim homeland in south Asia, marks the beginning of an election campaign likely to be bitterly contested. It was a chance for Khan to try to regain momentum lost to the PML-N, the faction of the Pakistan Muslim League led by former prime minister Nawaz Sharif.

A massive rally held by Khan in Lahore in late 2011 shocked the country's political establishment and prompted heady speculation that the former captain of Pakistan's cricket team, who currently has no seats in parliament, had a serious chance of becoming prime minister. But the PML-N responded to the challenge with a frantic spurt of activity, including a major public transport initiative in Lahore, intended to prove its ability to deliver major projects. Sharif is now most likely to head the biggest party after the polls in seven weeks' time.

Opinion polls give his party a clear lead over Khan and the Pakistan People's party (PPP), which has led a governing coalition since 2008. In another sign of the party's growing strength, Sharif has recently attracted a swath of defectors from other parties, including many political power brokers. "The politicos say we are on the back foot because some so-called electables have switched to the PML-N," said Shah Mehmood Qureshi, the vice-chairman of the PTI, speaking to the Guardian before the storm. "I think today's meeting will show ordinary citizens are desperate for change. They have seen a two-party system for decades but for the first time they now have a viable third option."

The party put enormous efforts into attracting as many people as possible to the event. The city was festooned with posters for the event, television and radio ads filled the airwaves and many complained of receiving a barrage of cold calls from party workers urging them to attend. Sceptics argue that Khan's celebrity and the ability to attract vast crowds to rallies may not be enough to confront the organisational strength of the established parties.

The PPP's chances of clinging to power are thought to be extremely slight, with the public likely to punish it for a dismal record in power. However, it may try to exploit its willingness to defy the threat of US sanctions with a deal to build a gas pipeline from neighbouring Iran in the dying days of the current government. "The pipeline is a great thing, but no one is going to thank the PPP," said Zian Shairazi, a young mechanical engineer, and one of the attendees at yesterday's Lahore rally. "They had five years to deliver this and they do it in their last days in office while the rest of the country is suffering power cuts."