A large number of devotees had already reached the shrine for darshan on the day of opening of the temple.
Security in and around the temple has been tightened in view of the heavy rush expected over the coming days.
The temple rituals would begin with the tantri performing the Ashtadravya Maha Ganapati homam tomorrow morning, the first day of the Malayalam month Virischikam, temple officials said.
The main Neyyabhishekom offering to the presiding deity will begin after the Ashtabhishekom.
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Today's programme started with chief priest (Tantri) Mahesh Mohannaru opening the 'sanctum sanctorum' of the temple in the evening.
A V Unnikrishnan Namboothiri is the new melsanthi (head priest) of the Lord Ayyappa temple while Aneesh Namboothiri is the melsanthi at the Malikappuram Devi Temple.
Their tenure is for a period of one year.
The 41-day Mandalam festival will conclude on December 26 after Mandala pooja.
The temple would be closed after the Athazhapuja in the evening on that day.
The temple would reopen for the Makaravilakku festival on December 30.
Several rounds of review meeting including chaired by state chief Minister Pinarayi Vijayan were held to review the eleborate arrangements and amenities made at the hill shrine situated in the Western Ghats in Pathanamthitta district.
The procession carrying the sacred attire, Thiruvabharanam, would set off from the Pandalam Valiyakoickal Sastha temple on January 12.
The temple would be closed a week after the Makaravillaku marking the culmination of the annual pilgrimage season.