(Recasts throughout with rally start, previous LAGOS)

By Camillus Eboh

ABUJA, May 1 (Reuters) - Thousands of people gathered in heavily guarded squares and stadiums in Nigeria's main cities on Tuesday to protest last month's flawed presidential election, which gave the ruling party a crushing victory.

Civil society groups had feared a security crackdown ahead of the rally in the capital Abuja called by powerful trade unions and main opposition parties after the electoral body declared Umaru Yar'Adua of the People's Democratic Party the winner of an April 21 presidential election.

Monitors said the vote was rigged. Yar'Adua says he won the election fair and square.

Armoured Personnel Carriers were stationed at the venue while troops patrolled the city. Security forces searched people and seized political placards before letting them through the gate. One placard read: "This election was a fraud, cancel it."

"Today is not for protest. We will not allow political placards, political rally, nor political statement," Lawrence Alobi, police commissioner for Abuja said.

The Inspector General of Police Sunday Ehindero said while May Day rallies would be allowed at designated grounds, anyone marching without a permit would be dispersed with teargas.

Despite the heavy security presence, more than 5,000 people gathered in Abuja to denounce the election as a fraud and just under that number mounted a similar demonstration in Lagos.

Any government based on the outcome of the elections "will be politically and morally disabled," said Abdulwaheed Omar, president of the influential Nigeria Labour Congress (NLC), at the May Day rally.

"Obviously our dear country is in a political logjam. That is why the NLC is making broad nationwide consultation with broad-based interest groups to fashion a way out of this crisis," Omar said.

FEAR OF CRACKDOWN

Former military strongman Muhammadu Buhari, presidential candidate of the main opposition All Nigeria People's Party and Vice President Atiku Abubakar of the Action Congress party did not attend the rally.

Opposition leaders were reluctant to call for mass protests because they fear the demonstrations could be taken over by violent or criminal elements and lead to a security crackdown.

Mabel Adinga Ade, head of the biggest local election observer group said secret police arrested at least one campaigner at the Abuja rally, while others received anonymous threats ahead of the event.

The group, which receives funding from the United Nations Development Programme, the European Union, Canada and Britain, had complained of intimidation from the secret police and the electoral agency before the elections.

The secretive State Security Service answers to the National Security Advisers which, in turn, is under orders from President Olusegun Obasanjo.

International monitors and local observers have denounced the presidential vote and state polls on April 14 as a failure, but the government said the criticism was intended to make way for a coup.

Obasanjo has told aggrieved parties to seek redress through the courts before he hands over power on May 29 in the first transition from one civilian leader to another in Africa's most populous nation.

The programme of non-violent protests started with prayer sessions on Friday. (Additional reporting Tume Ahemba)

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