Nititorn Lamlua, leader of the Network of Students and People for Reform of Thailand, demanded US ambassador Kristie Kenney come out and listen to protesters' reasons for wanting to remove caretaker Prime Minister Yingluck Shinawatra and push for reform before a general election, not after.
They handed out an open letter at the rally strongly attacking Western governments and media organisations for unfairly judging and reporting on the political situation in Thailand.
"Your views and reports on the situation in Thailand have been very negative and biased, with much prejudice," the letter said. "We want to inform leaders or representatives of those Western governments and others that their concern over the situation here is unwarranted. Matters in Thailand are none of their business," it added.
"If you continue to 'express concern' time and again, directly or indirectly, then we presume that your growing concern is mainly based on your self-serving interests and that certainly signify ills for Thailand through uninvited interference in the internal affairs of our country," the letter said.
The protesters also set up a flag pole and put up a national flag to send a message to Washington that the ongoing rallies are a domestic issue and other countries should not intervene.
Mr Nititorn on Sunday accused Washington of calling for democratic principles and human rights without even considering that Thais are fighting an undemocratic government.
The US State Department issued a statement in Washington last week supporting the attempt to solve the political impasse "peacefully and democratically'' following the prime minister's decision to dissolve the House of Representatives and call elections now set for Feb 2.