Addressing a press conference, Inspector-General of Police Ismail Omar warned that participants at the rally will be arrested.
"The organiser must comply with the Peaceful Assembly Act 2012. Marching without permit aimed at arousing anger is wrong according to the law," Omar said according to state news agency Bernama.
The opposition on its part argues it had complied with provisions of the Peaceful Assembly Act 2012 as it has informed the police about the proposed rally to be held at the Kelana Jaya stadium at 8.30 pm.
"It complies with the Peaceful Assembly Act - a permit is not needed for a rally, only the need to inform the police," he added.
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Urging the people to attend tonight's rally, the opposition has appealed to Malaysians to wear black to the assembly to express their objection to alleged electoral fraud in Sunday's 13th general election, online news portal Malaysiakini said.
Anwar has disputed the results of the election in which Prime Minister Najib Razak-led Barisan Nasional (National Front) coalition retained its 56-year grip on power, winning 133 of the 222 parliamentary seats.
He has accused the Election Commission of "being complicit in the worst electoral fraud in our nation's history."
However, Najib's office in a statement has dismissed opposition's claims of electoral fraud as "unsubstantiated.