The National Assembly Legislative Assembly passed the first reading of the amendment, which Human Rights Watch warned would give too much power to the military courts.
The amendment would authorise the military to detain civilians for up to 84 days compared to the current seven days.
Currently, however, the seven-day detention period can be reapplied for a period of up to 90 days.
Brad Adams, Asia director of Human Rights Watch, said the government was trying to give the military unchecked authority to detain civilians.
He said Thai lawmakers should reject this military power grab that would put all citizens at risk of prolonged detention without charge.
But Udomdej, who is also Deputy Defence Minister, said the increased power would be directed only at soldiers.
The amendment also adds more jurisdiction detail clarity, said Udomdej, adding that there was a necessity to amend several Army regulations since some dated back to 1947.
He said he hoped Human Right Watch better understood the issue after receiving an Army explanation.
Meanwhile, he said the arrest of anti-coup movement members on Saturday was in accordance with the law.
“There had already been warnings calling for those people to halt any activity that would breach the law,” he said, adding that several forums had been established for the public to air opinions.
The National Legislative Assembly will examine the proposed amendment on Thursday.