A judge rejected a request by the woman's husband to overturn an order by Florida governor Jeb Bush to resume feeding her.
The lawyer for the woman's husband said angrily yesterday that she was "literally abducted from her deathbed".
Experts said the US government's action raises legal issues that could complicate the case even further.
Terri Schiavo, whose feeding tube was removed last week, began receiving liquids intravenously on Tuesday after lawmakers rushed to pass a bill designed to save her life. A judge later rejected a request by her husband, Michael Schiavo, to overturn Mr Bush's order, at least for now.
"It was just an absolute trampling of her personal rights and her dignity," Michael Schiavo's attorney, George Felos, said on NBC's Today show. "We believe that a court sooner or later, we hope sooner, will find this law to be unconstitutional."
He also said Terri Schiavo was suffering signs of organ failure on Tuesday and the reintroduction of fluids in her system after a week without food or water could just make her suffer more before dying. A Morton Plant Hospital spokeswoman said yesterday she could not release any information on Ms Schiavo.
Observers wondered whether the legislature and the governor overstepped constitutional boundaries by ramming through legislation that overruled the courts.
"It presents a new legal issue that I've never heard of," said former Florida Supreme Court Justice Stephen Grimes. Former attorney general Bob Butterworth said the upcoming legal wrangling "could be fairly historic".
Ms Schiavo, 39, has been in a vegetative state since 1990, when her heart stopped because of a chemical imbalance.
Her parents, Bob and Mary Schindler, have fought to keep their daughter alive and say she still could recover. Her husband says she would rather die than be kept alive artificially, though her parents said she never told them of such a wish.
The feeding tube was removed last week after a court refused to intervene. Doctors had said she would die within a week to 10 days without nutrition and water.
An ambulance took Ms Schiavo from a Pinellas Park hospice to Morton Plant Hospital on Tuesday after Mr Bush issued his order to resume feeding her. A crowd cheered outside as she left.
"I'm ecstatic she's being fed again," said her brother, Bob Schindler Jr. "I don't think I can describe the way I feel right now. It's been unreal."
Suzanne Carr, the woman's sister, called the Senate's action "a miracle, an absolute miracle".
Her mother, Mary Schindler, broke down and cried after the Senate vote.
Hours earlier, the Senate voted 23-15 for legislation to save Terri Schiavo. Within minutes, the House voted 73-24 to send the bill to Mr Bush. The governor signed it into law and issued his order about an hour later.
"It's restored my belief in God," said Terri Schiavo's father, Bob Schindler.
Michael Schiavo, meanwhile, was "deeply troubled, angry and saddened that his wife's wishes have become a political pingpong," Mr Felos said.
Harvard law professor Laurence Tribe said the action by Mr Bush and the legislature "violates the core principles" of a 1990 US Supreme Court decision.
The court ruled in a Missouri case that Nancy Cruzan, who had been fed through a tube for seven years, could be permitted to die if "clear and convincing evidence" proved that was what she wanted. Her parents had fought for the right to remove the tube.
"I've never seen a case in which the state legislature treats someone's life as a political football in quite the way this is being done," said Mr Tribe.
Although the legislature acted swiftly, even some who supported the bill expressed concern about it.
"I really do hope we've done the right thing," said Senate President Jim King, a Republican. "I keep on thinking: 'What if Terri didn't really want this done at all?'"
During the years she has been in a vegetative state, her parents reported their daughter laughed, cried, smiled and responded to their voices.
Video showing the dark-haired woman appearing to interact with her family was televised nationally. But the court-appointed doctor said the noises and facial expressions she made were reflexes.
Both sides accused each other of being motivated by greed over a $1 million medical malpractice award from doctors who failed to diagnose the chemical imbalance which caused her heart to stop.