Mobile phone users in southern Thailand face having their lines disconnected as part of a drive to reduce bomb attacks.
Security forces hope the registration will cut bomb attacks
Thousands queued to register prepaid phones on Tuesday to beat a midnight deadline set by the government.
From midnight all unregistered phones will have their signals blocked, a move Thai officials hope will prevent their use as improvised bomb detonators.
About 1,000 people have died in the Muslim-majority south of Thailand since an insurgency broke out in early 2004.
An estimated 470,000 mobile phones in three provinces across the south of the country are thought to be affected.
The government order requires all owners of prepaid mobile phones in Narathiwat, Pattani and Yala to register by midnight on Tuesday (1700 GMT).
"There are three ways to detonate a bomb - by time-setting, by cable and by phone," said Thai Interior Minister Kongsak Wantana.
"Taking one of the three out of the way would help cut bomb attacks in the region."
Home to most of Thailand's 4% Muslim minority
Muslim rebels fought the government up to the mid-80s
Suspected militants have upped attacks since 2004, targeting Buddhists
Security forces' response criticised by rights groups
The signal blockage will also affect some customers across the Malaysian border.
Thais with phones registered on contract agreements have no need to register, as their details are already available to the government.
But prepay customers in the rest of the country will need to register their handsets by the end of the year, Mr Kongsak said.
Staff in post offices and at branches of mobile phone providers reported long queues on Tuesday as people finally realised the deadline was approaching.
An official at DTAC, Thailand's second largest mobile phone company, told the Reuters news agency that 800 people had queued at his office.
As many as 5,000 queued at the Narawithat provincial post office, the AFP news agency reported.
The three provinces have been placed under emergency law as part of a government drive to cut insurgent violence.
Authorities blame the regular attacks on criminal gangs, Muslim extremists or local corruption.