* Tunisia president empowered to rule by decree
* President can appoint cabinet, set policy
* Parts of constitution will not be in force
* Opponents call it a coup (Adds gazette entry, background)
By Tarek Amara and Angus McDowall
TUNIS, Sept 22 (Reuters) - Tunisia's president declared on Wednesday he can rule by decree and ignore parts of the constitution as he prepares to change the political system, prompting immediate opposition from rivals.
Kais Saied has held nearly total power since July 25 when he sacked the prime minister, suspended parliament and assumed executive authority, citing a national emergency in a move his foes called a coup.
The events have thrust Tunisia's young democracy into its biggest crisis since the 2011 revolution that ended autocratic rule and triggered the Arab Spring, despite Saied's pledges to uphold the freedoms won a decade ago.
As the weeks have passed, he has come under growing pressure from key Tunisian political players and Western donors to name a prime minister and explain how he intends to move past the crisis.
The presidency announced via social media that Saied had put in place new legislative and executive measures and would form a committee to amend the political system.
The social media messages said the preamble to the 2014 constitution would remain in force along with other parts of the document that did not contradict the new powers Saied had assumed.
The official gazette where all Tunisian laws are published showed a new entry giving Saied the right to issue "legislative texts" by decree.
The gazette entry also said the government, previously under a prime minister who answered to parliament, would report to the president, who would appoint cabinet members and set its policy direction and basic decisions.
The presidency also said parliament's activities would remain frozen with members still stripped of their immunity from prosecution
The leader of the moderate Islamist Ennahda party, the biggest in the deeply fragmented parliament and a member of successive governing coalitions, immediately rejected Saied's announcements on Wednesday.
Rached Ghannouchi told Reuters the announcement meant cancelling the constitution and that Ennahda, which had already declared Saied's July 25 intervention a coup, would not accept that.
A senior official in Heart of Tunisia, the second-largest party in parliament, accused Saied in a tweet of conducting a "premeditated coup." "We call for a national alignment against the coup," said Osama al-Khalifi, the official.
This month a Saied adviser told Reuters he was planning to suspend the constitution and offer a new version via a public referendum, prompting a backlash from the powerful labour union and political parties.
Saied has denied having dictatorial aspirations, insists his moves are constitutional and has vowed to uphold the rights of Tunisians.
His broadly popular intervention came after years of economic stagnation and political paralysis, aggravated by a sharp spike in COVID-19 cases and a day of violent protests.
However, as the weeks have passed a growing number of Tunisians have grown concerned by the lack of clarity on Saied's plans and the absence of a prime minister.
Rights groups have also pointed to the arrest of several parliament members and business leaders on a variety of charges, including some old ones that were reactivated after their immunity was removed.
One of the detained parliament members told Reuters on Wednesday he had been freed.
After criticism over the reported widespread use of travel bans against people in the political and business elite, Saied said last week that only people facing a judicial warrant or summons would be stopped from leaving Tunisia.
The first protest against Saied since his intervention took place on Saturday, and activists have called for a larger one this weekend.
(Reporting by Tarek Amara Writing by Angus McDowall Editing by Gareth Jones, Peter Graff, Steve Orlofsky and Cynthia Osterman)
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