Charles Clarke has come under sustained attack over his plans to merge 43 police forces in England and Wales to as few as 12 regional constabularies.
Charles Clarke plans a big shake-up of policing
The home secretary told MPs the new forces would be more efficient and help combat terrorism and organised crime.
But shadow home secretary David Davis warned that the changes would mean a less localised service.
Mark Oaten, the Lib Dems home affairs spokesman, accused the government of carrying out a "rushed process".
The plans were announced after a report by Her Majesty's Inspectorate of Constabulary (HMIC) said forces with fewer than 4,000 officers were not equipped to fight sophisticated modern crime.
Failures that led to the murders of Holly Wells and Jessica Chapman, in Soham, Cambridgeshire, had also led to criticism of communication between forces.
However, the Association of Police Authorities (APA) has accused ministers of trying to "bribe" forces into submission by offering extra cash to those who submit merger plans by Friday.
All forces have been asked to submit their final proposals for restructuring by the end of the year.
Advocates of the changes, the biggest shake-up of police in 30 years, say larger forces work better and the plans could save up to £2.3bn over 10 years.
However, in the Commons on Monday, Mr Clarke said police forces had to have the ability to fight terrorism, serious and organised crime and to handle major incidents.
The HMIC had concluded that "the current structure of policing, in their words, is no longer fit for purpose" and that strategic forces offered the best solution, he said.
"Faced with this clear, well-argued and independent police advice, I judged it essential to act rather than to ignore their proposals."
Mr Davis said: "Our concern is that the government is forcing it on police forces that do not want it and do not need it."
The process of amalgamation would be a "ferociously disruptive and distracting exercise" and set-up costs could be "astronomical", he said.
The APA estimates costs of up to £500m-£600m.
"It will be a tragedy if the government pushes through this hasty, ill-considered, costly, disruptive and dangerous plan," Mr Davis said.
Mr Oaten was critical of the "rushed process" of the consultation, the costs of setting up and running larger forces and whether it was the right way for the police to go.
Michael Howard, making his first backbench speech since relinquishing the Tory leadership, attacked plans to merge Kent Police with forces in Sussex and Surrey at an estimated cost of £58m.
The chief constable, Kent Police Authority and nine out of 10 Kent residents believed it would do best as a stand-alone strategic authority, said the former home secretary.
Labour's Gwyneth Dunwoody said she was "astounded" by the speed with which the government was trying to push the reforms through, adding that she had "grave doubts" that larger forces would work.
She also warned that the reforms would lead to a national police force - something Mr Clarke has denied.
Labour's Ashok Kumar said he had "serious concerns" about plans which were being "bulldozed" by ministers. He also could not understand the "bigger is better" thesis, he said.
Labour MP Paul Flynn said: "This is a decision that is rushed. It's not based on any rational basis. It will help only the criminals and it will harm the whole structure of the police. It's reorganisation for reorganisation's sake."
Ministers have told force leaders to borrow money to meet the merger costs.
But APA chairman Bob Jones has said no police authorities would agree to the plans unless Mr Clarke promised the government would meet all the merger costs.
"We will not be bullied or bribed into making decisions in three months about changes to our police forces which have done a good job for the last 30 years," he said.