Harare (AFP) - Zimbabwean President Robert Mugabe opened parliament Tuesday with a warning that locals who front for foreign firms trying to dodge indigenisation laws face "decisive action".

"It is depressing that some of our people have turned themselves into mere fronts of foreign investors," he said, doubling down on controversial rules that bar foreign companies from holding majority stakes.

"Decisive action shall indeed be taken to address these negative developments and to strengthen the general enforcement of the relevant legislation," he said.

In his near half-hour address the 90-year-old veteran leader avoided talking about rampant speculation over who will succeed him, amid increasingly visible splits in his ZANU-PF party.

Instead he attacked foreign sanctions and indigenisation -- topics which have been the bread and butter of his oratory in recent years.

In 2007, Zimbabwe enacted a so-called "indigenisation law," forcing foreign-owned firms to hand over at least 51 percent of shareholding to local partners.

Mugabe said the law, which followed controversial land seizures launched in 2000, was meant to reverse imbalances which resulted from colonial rule.

The law has forced mining firms, including the country's biggest platinum miner Zimplats and international banks such as Ecobank, to cede 51 percent of their shares to locals.

Critics say the law benefits Mugabe's close allies while scaring off foreign investors at a time the country is in dire need of investment and is experiencing a liquidity crunch.

Zimbabwe police on Tuesday detained three protesters calling on Mugabe to step down over his government's failure to fix the economy, a lawyers' group said.

"Three activists were picked up by riot police during a peaceful demonstration and they are currently detained," Kumbirai Mafunda, spokesman for Zimbabwe Lawyers for Human Rights told AFP.

Mafunda said the trio -- including journalist-turned-activist Itai Dzamara -- had earlier been beaten up by Mugabe supporters, who gathered in a public square as Mugabe opened parliament.

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For over a week the activists have been staging a protest in a municipal park adjacent to parliament brandishing banners demanding Mugabe's resignation, the lawyers said.

"Failed Mugabe Must Step Down" read one banner while another said "Corruption has killed the Economy."

Police spokespeople were not available to confirm the detention or if charges had been laid.

Previous demonstrations against Mugabe's government have been brutally put down by the security services.