Detroit's bankruptcy proceedings have met a wall of stiff opposition from public labour unions, creditors and retiree groups, who are trying to block the US city's move claiming it breaches the country's cherished constitution.

The number of court filings objecting to the bankruptcy is expected to hit 100, with some attacking Detroit's decision as a violation of the US constitution at a state and federal level.

US judge Steven Rhodes will hold a trial on 23 October to hear arguments for and against Detroit's bankruptcy, before making his decision on whether the insolvency proceedings can continue. Judge Rhodes had set 19 August as a deadline for filing objections.

An emergency services union attacked Detroit's emergency manager Kevyn Orr, appointed in March to supervise the city's financial operations, for allegedly not entering into serious negotiations with creditors to reach a deal that would have avoided bankruptcy.

Before a judge will start proceedings, the federal bankruptcy code obliges Detroit to prove that it is not only insolvent, but had also attempted good faith negotiations with creditors.

One public sector workers union argued that Chapter 9 of the federal bankruptcy code, which protects a municipality from its creditors, is in contravention of the US constitution, reported Reuters.

Attorneys of the city's largest union, the Michigan chapter of the American Federation of State, County & Municipal Employees, claimed Detroit designed an "unconstitutional, unlawfully authorized Chapter 9 proceeding seeking the haven of bankruptcy to illegally attempt to slash pension and other post-employment benefit obligations and cram such reductions down the throats of current and former city employees."

A similar filing argued that Chapter breached states' rights, said Reuters, while others claimed Detroit's bankruptcy would harm their retirement benefits, which would be unconstitutional.

Detroit, once America's automotive powerhouse, has become the largest city in US history to file for municipal bankruptcy, owing $18.5bn (£12.1bn, €14.1bn) to thousands of creditors.

The list of creditors, including present and past workers, spans 3,504 pages. The largest in that pack are two pension funds that have claims worth $3.74bn.

If the court does conclude that the city qualifies for bankruptcy, Detroit would have until 1 March 2014 to file a reorganisation plan, according to Judge Rhodes' schedule.