The group made its announcement in defiance of Washington's support for Thailand's caretaker government.
Anucha Ramanyanand, deputy spokesman of the Centre for the Administration of Peace and Order (Capo), yesterday said the Capo had worked with embassy officials on security operations in case protesters converge at the premises.
The move came after Nititorn Lamlua, leader of the anti-government Network of Students and People for Reform of Thailand (NSPRT) announced on Saturday night he would lead protesters to seize the US embassy.
"Listen carefully. We will storm and seize the [US] embassy," he told the crowd who had rallied at Chamai Maruchet bridge. Mr Nititorn, however, did not set a date for the demonstration.
He said Washington always calls on Thais to respect democratic principles and human rights without considering that Thais are fighting an undemocratic government.
The US government's support could lend the Yingluck Shinawatra government legitimacy, he said.
Pol Maj Gen Anucha said that after the NSPRT leader announced the planned seizure, the Capo contacted embassy staff and confirmed that police were on standby to offer protection if needed.
The NSPRT stayed put at its Chamai Maruchet protest base near Government House yesterday and did not move to the embassy as planned.
The US government earlier last week urged Thailand to work out its political differences in a peaceful and democratic manner.
"Prime Minister Yingluck has called for elections as a way out of political tension and demonstrations," it said.