DMK working president MK Stalin demanded resignation of Chief Minister Edappadi K Palaniswami and Speaker Dhanapal.
Stalin, who is Leader of Opposition in the Assembly, described the disqualification as "a brutal murder of democracy."
He said "Speaker P Dhanapal has lost the moral right to continue in his office," and demanded his "immediate resignation."
Also, he wanted Palaniswami to voluntarily quit office for allegedly trying to hold a floor test through "horse-trading," and "illegal disqualification."
In a statement, the DMK leader said the disqualification showed the "cowardice," of the Palaniswami regime in facing the House and winning (floor test) and said the government will be defeated in the people's court.
He said the government is in a situation to prove its majority and the present action was a "short-cut" towards achieving it.
According to the Constitution, Stalin said, only if an MLA leaves the party or if he voted in House violating the party whip, could a member be disqualified.
Disqualifying the 18 MLAs for petitioning Governor Ch Vidyasagar Rao showed the "unabashed enactment of the murder of Parliamentary democracy," he said.
The action against the 18 MLAs, who had revolted against Palaniswami last month, was taken under anti-defection and disqualification rules of 1986, said a statement from Assembly Secretary K Bhoopathy.
Stalin claimed that the disqualification went against the anti-defection law and a verdict of the Supreme Court.
He said the apex court had held in a case involving former Karnataka Chief Minister BS Yeddyurappa that (MLAs) telling the governor that they do not have faith in chief minister does not become "defection."
He said "the Speaker was not worried," about a petition given by ruling party (Dinakaran loyalists) MLAs seeking disqualification of O Panneerselvam and his supporters who had voted against the Palaniswami regime in March.
Tracing the month-long political turbulence in Tamil Nadu which began with 19 ruling party MLAs submitting a plea to the governor seeking the ouster of Palaniswami, he said the Centre and Rao should own "full responsibility" for the "extraordinary political situation," in Tamil Nadu.
Later, speaking to reporters, he said a meeting of DMK legislators will be held tomorrow to discuss the development.
To a question on convening an all-party meeting and going to the people's court on the issue, he said "expect it...we will invite you."
Meanwhile, Dinakaran alleged that the disqualification was "a bid to to demonstrate majority (in the Assembly) through short-cut."
He recalled that his faction has been maintaining all along that the Palaniswami regime does not enjoy majority.
Stating that his loyalist MLAs will move the court on the issue, he said legislators of his camp will participate in voting (in the event of a floor test) in the House.
Tamil Nadu Congress Committee President Su Thirunavukkarasar said the government cannot continue without proving majority.
Stating that the disqualification may not stand legal scrutiny, he said his party will deliberate with allies including DMK on the matter.
PMK chief S Ramadoss described the disqualification as "murder of democracy".
The action against the 18 MLAs will not "match" with any provisions of the anti-defection law, he said.
The action taken by the speaker on a plea of government chief whip S Rajendran was "flawed," he said adding MLAs can be disqualified only if they act against the whip in the House.
He said the "people desired" that the government should immediately quit.
"This is not shocking. This is being done to retain power," said VCK chief Thol Thirumavalavan.
CPI (M) State Secretary G Ramakrishnan said the disqualification has "ulterior political motives and against democracy."
MLA and ally of AIADMK and M Thamimun Ansari (Manithaneya Jananayaga Katchi) said "the 18 MLAs did not join any other political party."
The people viewed it as a "murder of democracy," he said.
He wanted "reconsideration," of the decision to disqualify them.