Campbell Corrigan, acting chief constable of Strathclyde Police, said it may mean there would not be enough solicitors on duty to provide cover when an accused person is arrested and taken to a police station.
Defence lawyers will hold a demonstration outside Holyrood today as MSPs debate plans to make suspects pay a contribution toward their legal aid costs, to be collected by solicitors.
Hundreds of members of both the Glasgow and Edinburgh Bar Associations are prepared to go on strike over the plans, which they say will throw the criminal justice system into chaos.
Mr Corrigan warned any action would have huge consequences and would affect the number of suspects that could be kept in custody.
He said: "In practical terms, if there was a strike we would have to reassess how many people we were keeping for the court. We would have to use the powers we have for 'undertakings to appear' and 'liberated for report'."
Referring to the Cadder legal ruling, which has left police unable to interview suspects without a solicitor, he said: "It would be for the Crown to direct us in terms of the law for Cadder interviews if defence solicitors were not available to turn up. There will always be somebody, but it might take longer.
"It is not a particularly nice thought that we would get to that stage. It would not be good from anybody's point of view, to be honest.
"If we have a strike we will deal with it. We will contingency-plan as we always do for dealing with accused."
Critics have warned the moves will turn lawyers into unpaid debt collectors and raised fears some cases will be derailed if the accused cannot pay.
Under the plan, those accused in summary, or non-jury, criminal cases who have a disposable income of at least £68 a week – or £750 in savings – would be asked to make a contribution.
Such legal aid is means-tested and many already pay contributions towards fees, but all money is collected by the Scottish Legal Aid Board, instead of lawyers.
Ann Ritchie, Glasgow Bar Association vice-president, said lawyers would face a conflict of interest if faced with a client previously unable to pay contributions. She said: "This is not about lawyers getting their fees, it's about ensuring that justice is accessible to all.
"This is the concern of everybody in the Scottish Criminal Justice System, and these proposals will bring chaos if they are implemented. It is the Scottish Legal Aid Board's job to administer fees. We do not have the ability to enforce payment."
Ministers say it would save nearly £4 million a year and only one in five cases would be affected. The Crown Office declined to comment.