The rally is being held on the eve of Opposition Leader Anwar Ibrahim's deadline to take power through defections from the ruling coalition, according to a Malaysakini report.
The opposition alliance said it expected some 30,000 supporters to gather for the rally in Kelana Jaya stadium on Monday night to call for the release of the two still detained, after the journalist was freed Saturday.
It will be held on the eve of September 16, Anwar's deadline to seize power and also the day in 1963 when Sabah and Sarawak joined together with Malaya to form modern Malaysia.
"We gained independence but Malaysians continue to live under repression. We believe Malaysia Day should symbolise freedom, justice and equality," said PKR information chief Tian Chua.
"The gathering at Kelana Jaya stadium is to express our solidarity for the two people held under the draconian security law and to demand their freedom," he told AFP.
Rights groups have condemned the use of the Internal Security Act (ISA), which allows for indefinite detention without trial, and the United States summoned
Malaysia's top envoy in Washington in protest over the arrests. - Extracts from
DAP MP Teresa Kok was arrested over allegations she complained about the noise of morning prayers at a mosque.
The other detainee is controversial blogger, Raja Petra Kamaruddin, 58, who has repeatedly targeted government figures on his website Malaysia Today.
He has already been charged with sedition and defamation after linking Deputy Prime Minister Najib Razak and his wife to the sensational murder of a Mongolian woman.
Police has since released Sin Chiew daily reporter Tan Hoon Cheng after detaining her more than 16 hours under the ISA over her story on suspended Umno man Ahmad Ismail's "Chinese were squatters" remarks.
Ahmad was subsequently disciplined and the decision to target Tan caused widespread dismay and disbelief including from Chinese-based parties in the coalition.
Read rocky'sbru's entry: Malaysia Day party at Kelana Jaya tonight here.