* Packed stadium shows Buhari's northern support

* Former military strongman pledges tighter security

* Opposition hope to force President Jonathan to run-off

(Repeats story from March 2 with no changes)

By Sahabi Yahaya

KADUNA, Nigeria, March 2 (Reuters) - Thousands of Nigerians flocked to see former military ruler Muhammadu Buhari launch his campaign for April elections on Wednesday, underlining the grass roots support he commands in the mostly Muslim north.

Supporters were crammed shoulder to shoulder in a sports stadium in the northern city of Kaduna as Buhari, President Goodluck Jonathan's main election rival, took to the podium pledging better security, less corruption and more jobs.

Dressed in a blue agbada -- traditional robes -- he said Africa's most populous nation needed change after 12 years of domination by the ruling People's Democratic Party (PDP), which has won all three presidential races since military rule ended in 1999, each of them marred by intimidation and fraud.

"The votes which we are going to cast in the upcoming election may amount to a worthless exercise if we are not going to defend them and ensure they count," Buhari said in the Hausa language most widely spoken across northern Nigeria.

"Our civic duty as citizens would be accomplished if after we cast our votes, we stick around to defend them, and ensure they count and are counted," he said to cheers from the crowd.

Ballot boxes were whisked away from polling booths at gunpoint in some areas during previous Nigerian elections and stuffed with forged votes, just one of several acts of fraud which led observers to deem them not credible.

Buhari told Reuters in an interview last month that Nigerians, spurred on by popular uprisings which have toppled autocratic governments in Tunisia and Egypt this year, would not stand for another rigged vote. [ID:nLDE71K0MY]

The former military strongman nonetheless faces a tough challenge unseating Jonathan, who as the incumbent sits atop a vast patronage system greased by oil revenues which enables him to command loyalty and makes him the front-runner.

Buhari ruled Nigeria between December 1983 and August 1985, an iron-fisted administration best remembered for its "War Against Indiscipline", a campaign against graft in which politicians were jailed and drug traffickers executed.

His reputation as a disciplinarian won respect from many ordinary Nigerians, who lament the extent to which corruption has become endemic, but the resentment of the political elite.

HOPES FOR A RUN-OFF

Buhari pledged a tough line on armed robbery, religious killings and kidnapping which have blighted parts of Nigeria but also promised to reform a police force renowned for extracting bribes, arbitrary detentions, and an inability to prevent crime.

He slammed the PDP's record on public finances in Africa's biggest oil and gas producer, saying the country's foreign reserves of around $35 billion had fallen 43 percent over the past 18 months despite rising world energy prices.

"(This) unbridled depletion , is not only worrisome, but a confirmation of the fact that Nigeria truly runs the most expensive, extravagant, ineffective and unproductive government in the world," he said, as supporters stood on car roofs to try to see in over the crowds.

Although the main opposition parties, including Buhari's Congress for Progressive Change (CPC), have not formed a formal alliance to try to unseat the PDP, they hope their regional strengths could combine to force a run-off.

While the CPC is strong in the north, the Action Congress (ACN) opposition party is dominant in the southwest, including the commercial capital Lagos, where Jonathan managed only to half-fill a stadium at a campaign rally on Wednesday.

Officials at the event in Lagos' Tafawa Balewa Square had to ask supporters sheltering from the scorching sun to come down from the terraces and surround the podium to give the impression of a better-attended event. Many left before Jonathan spoke.

To win victory in the first round, Jonathan must clinch the most votes overall but also gain at least 25 percent of the franchise in two thirds of the country's 36 states.

A strong showing in the north by Buhari and in the southwest by former anti-corruption tsar Nuhu Ribadu, the ACN's presidential candidate, could prevent him from doing this and force a run-off in which he may face a united opposition. (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/ ) (Additional reporting by Tume Ahemba in Lagos; Writing by Nick Tattersall; editing by Tim Pearce)

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