Frontline police awaiting deployment to Saturday's demonstration against government cuts have appealed for clear instructions about what to do in the event of disorder.

Peter Smyth, chair of the Metropolitan Police Federation, told the Guardian that operations – which have degenerated into pitched battles between demonstrators and the police – have been hampered by ambiguity as orders are relayed down the chain of command.

About 4,500 police officers are being assigned to the demonstration, which is expected to be the biggest mass protest the UK has seen since the march against the war in Iraq in 2003. Off-duty officers are also expected to join the march to protest about planned cuts of 20% to police budgets.

"If you are the officer and the order comes down to clear the streets, that can mean going up to people and saying: 'Excuse me, it is time to go, and can you move along please'. Or it can mean a baton charge. Sometimes that is not clear," Smyth said. "Officers need to be given clear instructions about what is expected of them. That is a problem, and senior officers have been a bit guilty of issuing ambiguous orders."

On the Guardian Focus podcast, Smyth said that officers are hoping for better this time. The Met was particularly stung by criticism after rioters at last November's student march were permitted to cause damage and invade Conservative party headquarters at Millbank. "The Met says it has listened. Clearly Millbank was a complete disaster for the Met. They underestimated the protest and weren't ready when violence broke out. I think this time they will be ready, and I'm really hoping it will be a peaceful protest," he said.

Smyth said some members will attend the demonstration while off duty. "There will be a number of police officers joining the march so we have a bit of a common cause with the TUC on this one," he said.

The federation has previously voiced concern for its members when violence breaks out. In January, an editorial in its magazine, Metline, demanded immediate answers to "some pretty fundamental" questions.

"Does the requirement to facilitate peaceful protest override the police's duty to prevent disorder? How many police officers are allowed to be injured before those who are left are deployed with the correct protective equipment? How much criminal damage is acceptable before the police are allowed to act?"

The editorial called for those in charge to set an example. "It is time for the leadership of the police service to shove their heads above the parapet and start leading; telling those officers who will be policing these protests whether to stand back and keep their fingers crossed or come forward and ensure that the law is upheld."

Assistant commissioner Lynne Owens, responsible for public order policing at Scotland Yard, said officers from the Met and those seconded from forces in the home counties will have a full day's briefing. "Our expectation is that they come expecting to police a peaceful protest, but if we have to intervene against violence I am confident that officers will do that. There is an expectation that they will use force if they have to, and it will be the minimum force required to meet our lawful aims." She said officers will be fully aware of what is expected. "I am very confident that because of the planning and conversations we have had, [all], down to the constable on the ground, will be very clear about expectations on the day."

The police operation is to be scrutinised at close quarters by the human rights group Liberty, whose officials have been allowed to sit in with police and TUC representatives during the weeks of planning. Liberty will also line the route with 100 legal observers and will have access to the police control room. During the march itself Liberty says the agreement will allow it to better fulfil the monitoring function, but the move has attracted criticism. Activists within the Network for Police Monitoring accuse Liberty of undermining its own independence. The network plans to deploy its own group of trained legal advisers to monitor events and provide advice to demonstrators on the day.