JAKARTA - Indonesia's main stock index slumped to its intraday lowest in more than two months on Thursday after the governor of Jakarta said he would reinstate social distancing restrictions due to a rise in coronavirus infections.

Indonesia's main stock index fell 4.1% within the first five minutes of trade.

Jakarta's governor Anies Baswedan said late on Wednesday the "wide-scale social restrictions" will start on Sept. 14 and are needed because the capital's health system is facing serious strain. Indonesia has the highest death toll from COVID-19 in Southeast Asia. 

The restrictions will be set up in stages and further details will be announced later, Baswedan added.

The move was "unexpected" but local companies were better prepared to deal with curbs this time around, brokerage Trimegah Securities said in a note.

The rupiah IDR= weakened as much as 0.7% to touch the weakest level since May 18, but has since regained some of its losses.

Bank Indonesia intervened in the foreign exchange market on Thursday to stabilise the currency, Nanang Hendarsah, head of monetary management told Reuters.

Official forecasts for Indonesia's 2020 GDP range from a contraction of 1.1% to a growth of 0.2%. Finance Minister Sri Mulyani Indrawati previously said a second wave of infections or the resumption of restrictions could hurt the outlook even more.

"It is too early to foresee any immediate implication to monetary policy, but a retightening of restrictions could cast clouds on the financial sector," Citibank said in a note.

(Reporting by Fransiska Nangoy, Gayatri Suroyo Additional reporting by Nikhil Kurian Nainan; Editing by) ((Fransiska.Nangoy@thomsonreuters.com; +62 21 2992 7610; Reuters Messaging: fransiska.nangoy.thomsonreuters.com@reuters.net))