Major roads will be shut and public transportation suspended from midnight (1600 GMT).
A group calling itself the Bersih 2.0 coalition plans to hold the rally in a sports stadium on Saturday.
Police have questioned more than 150 activists in recent weeks, and 91 have been barred from the city.
More than 30 activists remain in detention after being arrested almost two weeks ago.
The demonstrators initially organised a street rally, but agreed to hold their gathering in a sports stadium after objections from the authorities.
But police say the rally is still illegal, despite its change of venue.
"Merdeka Stadium, which they chose, is too close to the city centre and can create massive traffic jams and disrupt businesses," federal police chief Ismail Omar was quoted as saying by the New Straits Times.
Police have not given a permit for the rally, but the demonstrators have argued that they do not need permission.
"We are heading for the stadium," a representative of Bersih, Andrew Khoo, told AFP news agency.
"We will keep our focus on the basic demands... to campaign for free and fair elections. We won't be distracted."
Rally organisers say Malaysia's electoral system is plagued with fraud - they want longer campaign periods, automatic voter registration and equality of access to the largely government-linked mainstream media.
The authorities say the protesters are trying to promote communist ideology, thereby "waging war against the king".
Police have accused several of those they have arrested of carrying flyers and T-shirts with "inflammatory slogans".
The police launched a crackdown after a similar demonstration in 2007.
Analysts say that protest helped the opposition win an unprecedented number of seats in the last general election.
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