Karpal, who is the chairman of the opposition Democratic Action Party (DAP), faces up to three years in jail if convicted.
Insisting that he was voicing his legal opinion, he told reporters on Wednesday that he "can't see anything there which is seditious".
"These sedition charges against Karpal are utterly baseless. This is just an excuse to remove a powerful political opponent"
Elaine Pearson, Human Rights Watch
"In any event, sedition is an outdated piece of legislation," he said. "Of course I'm confident I've done nothing wrong."
A three-party opposition coalition known as the Pakatan Rakyat, or People's Alliance which includes the DAP, won political control of Perak in the March 8 general elections.
But the subsequent defections of three state legislators tipped the balance in favour of the ruling Barisan Nasional, the coalition in control of the federal government.
Following the defections, Sultan Azlan Shah, the titular head of the Perak state, formally recognised the BN as the state government.
Prosecutors say Karpal's statement questioning the sultan's mandate to appoint a new state administration during a power struggle in Perak was seditious.
The government has denied the charge is politically-motivated, saying it plans to call up to 20 witnesses mainly journalists who were at the press conference.
A reporter from the Utusan Malaysia, a government-controlled Malay-language daily, was called as the first witness on Wednesday.
Human Rights Watch has criticised the BN for trying to using the Malaysian legal system "to silence and punish its critics".
"These sedition charges against Karpal are utterly baseless," said Elaine Pearson, the rights watchdog's deputy Asia director.
"This is just an excuse to remove a powerful political opponent."