Kenyan President William Ruto vowed to take a tough line against "violence and anarchy" on Tuesday, after protests against his government's proposed tax hikes turned deadly and demonstrators ransacked parliament.

Mainly youth-led demonstrations had been largely peaceful as they grew over the past week but chaos erupted in Nairobi on Tuesday, with crowds throwing stones at police, pushing past barricades and entering the grounds of parliament.

The military has been deployed to support police who fired tear gas, water cannon, rubber bullets and -- according to a rights group -- live ammunition against protesters.

Five people were shot dead and 31 wounded, several NGOs including Amnesty Kenya reported in a joint statement.

"We shall provide a full, effective and expeditious response to today's treasonous events," Ruto told a press briefing in Nairobi, saying the demonstrations were "hijacked by dangerous people".

It was inconceivable that "criminals pretending to be peaceful protesters can reign terror against the people, their elected representatives and the institutions established under our constitution and expect to go scot-free," Ruto added.

"I hereby put on notice the planners, financiers, orchestrators, abetters of violence and anarchy."

The United States appealed for calm and 13 Western nations -- including Canada, Germany and Britain -- said they were "especially shocked" by the scenes outside parliament.

UN chief Antonio Guterres was "deeply concerned" by the violence and "saddened" by the reported deaths and injuries, his spokesman Stephane Dujarric said.

African Union Commission chief Moussa Faki Mahamat also expressed his "deep concern" and called on the country to refrain from further violence.

- 'Unleashed brute force' -

Outrage over proposed tax hikes and simmering anger over a cost-of-living crisis fuelled rapidly growing demonstrations that have caught the government off guard.

"This is the voice of the young people of Kenya," said Elizabeth Nyaberi, 26, a lawyer at a protest. "They are tear gassing us, but we don't care."

"We are here to speak for our generations and the generations to come," she added.

Amid the clashes, global web monitor NetBlocks reported that a "major disruption" had hit the country's internet service.

In the aftermath of the parliament compound breach, local TV showed images of ransacked rooms with smashed windows, while cars parked outside were vandalised and flags destroyed, according to an AFP reporter.

The governor's office in Nairobi City Hall -- just a few hundred metres from parliament -- was set alight, footage on privately owned Citizen TV showed, with a water cannon attempting to douse the fire.

After reports that live ammunition was fired at protesters, Kenya's main opposition coalition, Azimio, said the government had "unleashed brute force on our country's children".

"Kenya cannot afford to kill its children just because the children are asking for food, jobs and a listening ear," it said in a statement.

The military deployment was "in response to the security emergency" across Kenya, Defence Minister Aden Bare Duale said in a statement.

Despite the heavy police presence, thousands of protesters had earlier marched peacefully through Nairobi's business district, many live-streaming the action as they sang and beat drums in their push towards parliament.

Crowds also marched in the port city of Mombasa, the opposition bastion of Kisumu, and Ruto's stronghold of Eldoret, images on Kenyan TV channels showed.

- Protesters 'abducted' -

Amnesty International's Kenya chapter posted on X Tuesday that "the pattern of policing protests is deteriorating fast", urging the government to respect demonstrators' right to assembly.

Rights watchdogs have also accused the authorities of abducting protesters.

The Kenya Human Rights Commission said the abductions had mostly occurred at night and were "conducted by police officers in civilian clothes and unmarked cars", calling for the "unconditional release of all abductees".

Police have not responded to AFP requests for comment on the allegations.

The cash-strapped government agreed last week to roll back several tax increases.

But it still intends to raise other taxes to fill the void left by the changes, including on fuel prices and export taxes, saying they are necessary for filling the state coffers and cutting reliance on external borrowing.

Critics say the move will make life more expensive in a country already saddled with high inflation where well-paid jobs are out-of-reach for many young Kenyans.

Kenya has one of the most dynamic economies in East Africa but a third of its 52 million people live in poverty.

The country has a huge debt mountain whose servicing costs have ballooned because of a fall in the value of the local currency over the last two years, making interest payments on foreign-currency loans more expensive.

After the government agreed to scrap levies on bread purchases, car ownership and financial and mobile services, the treasury warned of a budget shortfall of 200 billion shillings ($1.56 billion).

© Agence France-Presse