Tablighi Jama'at is a Muslim missionary movement that was set up in India in the late 1920s.
Little is known about its funding and structure, but the United States-based think-tank Stratfor estimates it has 25 million to 80 million members. It is led by the New Delhi-based Maulana Saad Kandhalvi. Tablighi Jama'at - Society for Spreading Faith - preaches that Muslims should replicate the life of Prophet Muhammad and that it is Muslims' duty to travel around the world to convert non-believers to "the one truth faith".
The group is known as Jemaah Tabligh in Malaysia and Singapore.
Its most devout members have been known to follow the Prophet in all his customs - going as far as to eschew beds for sleeping and using twigs for teeth cleaning.
Members follow certain scriptures and practices like the "chilla" - a 40-day preaching tour which all are obliged to undertake annually - similar to a sufi order.
Tablighi Jama'at shuns the outside world and creates an atmosphere of spirituality, solidarity and purpose among members.
The group went to Malaysia in the early 1980s, and appealed particularly to workers in the industrial zones of states like Johor, Selangor, Penang and Perak.
Surfacing in Indonesia during president Suharto's time, it thrived there as it focused on faith renewal and shunned politics.
Seen to be non-political, Tablighi Jama'at spread fast in Malaysia and Indonesia in the 1970s and 1980s, when overtly political Islamist movements were closely monitored in Malaysia, and banned and persecuted in Indonesia.
Today, the group has spread to the Philippines, Singapore, Thailand, Vietnam and Cambodia.
Mr Macabangkit Lanto, a former Philippine ambassador to Cairo, said it is "catching fire".