Adam Adli, 24, was detained after speaking at a public political forum following the 5 May polls.
Malaysia's ruling Barisan Nasional (BN) coalition secured a simple majority in the national elections.
But it was their worst election result ever, and the closest poll since Malaysia gained independence in 1957.
The government said that the elections were free and fair, but the opposition said the results were marred by fraud, alleging multiple irregularities.
Mr Adli was arrested after he reportedly told members of the forum to "go down to the streets to seize back our power", AP news agency reported.
