KUWAIT (Reuters) - Kuwait's opposition on Monday condemned what it said was the state's use of excessive force against demonstrators and called for the release of those detained in Sunday's mass protest against a move to change the Gulf Arab state's electoral law.

Police in Kuwait used teargas, stun grenades and baton charges on Sunday to disperse tens of thousands of demonstrators protesting against changes to the voting mechanism, which the opposition has called a constitutional coup by the government.

A medical source said at least 29 people had been admitted to hospital, and at least 15 people, including two former members of parliament and a reporter, were detained. An opposition activist estimated that up to 50,000 people had gathered in different locations.

"The authorities revealed yesterday the ugly, authoritarian face of autocracy in its crass way of dealing with the peaceful popular march," Kuwait's opposition factions, including the country's Salafist Movement, said in a joint statement.

In a statement issued by the cabinet on Monday, Kuwait's Minister of Interior said the protest, which had been declared illegal, had been handled with "wisdom, self-control and tolerance."

The opposition decided to take to the streets after the government - which is dominated by the ruling Al-Sabah family - announced last week it was calling elections for December 1 and would change the electoral law "to preserve national unity".

The announcement was the latest move in an intensifying power struggle between the ruling establishment and parliament that has seen eight governments come and go since the emir, Sheikh Sabah al-Ahmad al-Sabah, came to power in 2006.

The 83-year-old emir dissolved parliament on October 7. It was the sixth time the oil-rich state and key U.S. ally had disbanded its legislature since early 2006.

Its oil wealth and a generous welfare state have helped Kuwait avoid the kind of "Arab Spring" protests that toppled leaders elsewhere in the region.

Reforms should lead Kuwait towards "a nation with a democratic parliament and the refusal of repression and autocracy," the opposition said in the statement.

A reference in the statement to the government's use of "foreign elements" in its special forces was vehemently denied by officials.

"I condemn the use of such allegations by the opposition. Why would the interior ministry use foreign elements if it has its own sons to help keep the peace," a source in the ministry told Reuters.

Kuwait's opposition has been emboldened after Arab protests toppled four heads of state.

Some in the opposition have been demanding a constitutional state and for the emir to resign but for governments to be formed by majority groups in the 50-member assembly. Others have made more modest demands for the government formed by the emir to be subject to full parliamentary scrutiny.

(Reporting by Ahmed Hagagy, Writing by Raissa Kasolowsky; Editing by Michael Roddy)