Shops and other buildings have been torched in the latest unrest

Water cannons

"We are a peaceful people who do not want violence," said a top opposition official, asking people to disperse.

The Kenyan opposition leader, Raila Odinga, had called for a million people to gather in the city's Uhuru Park.

More than 300 people have been killed and at least 70,000 driven from their homes across Kenya since Sunday.

Orange Democratic Movement (ODM) official William Ruto told reporters the mass rally would be held on Friday instead.

This is a country that has been held up as a model of stability. This picture has been shattered

Desmond Tutu

South African Archbishop

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South African Archbishop Desmond Tutu has flown in to try to mediate in the crisis, triggered by claims of vote rigging in the 27 December presidential election.

The Nobel laureate is expected to meet Mr Odinga but government officials said Mr Kibaki, 76, had no plans to meet him.

Archbishop Tutu was quoted by Reuters news agency as saying: "This is a country that has been held up as a model of stability. This picture has been shattered."

Thousands poured out of the pro-opposition Kibera slum and other shanty towns after dawn on Thursday to head for the planned rally in the centre of Nairobi.

But crowds of Kibera residents were blocked by riot police and a tense stand-off ensued.

Stock exchange shuts

Security forces also surrounded Uhuru (Freedom) Park and blocked at strategic spots to keep away demonstrators.

Wearing white scarves and singing Kenya's national anthem, some protesters got within a mile of the city centre.

They stopped traffic by sitting down in a main road, prompting police to disperse them with water cannon and tear gas.

It is a defining moment for the country because, as you can see, the people of this country are not willing to take this just lying down

Raila Odinga

Opposition leader

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The latest disorder prompted the Nairobi Stock Exchange to close barely an hour after opening.

The BBC's Odiambo Joseph in Mombasa says the coastal city has not escaped the disorder, witnessing running battles on Thursday morning between police and rowdy youths.

Mr Odinga, 62, had vowed to press ahead with the rally, saying it would send a peaceful message to opposition supporters.

He told the BBC: "It is a defining moment for the country because, as you can see, the people of this country are not willing to take this just lying down."

But Vice-President Moody Awori called on Mr Odinga to cancel the protest, urging him in a public appeal: "Overcome your anger, your bitterness and all negative emotions for the sake of our country."

Supporters of President Kibaki (a member of Kenya's predominant Kikuyu tribe) and Mr Odinga (from the Luo community) have accused each other of genocide and ethnic cleansing in the post-poll unrest.

Perhaps the most shocking of the violence in the past week was the torching on Tuesday of a church at Eldoret, in western Kenya's Rift Valley, where at least 30 (mainly Kikuyu) people were burnt alive.

Mr Kibaki invited all MPs to crisis talks at state house on Wednesday but Mr Odinga's Orange Democratic Movement refused and demanded the president quit.

African Union chairman, Ghanaian President John Kufuor, had been expected to visit Kenya on Thursday but Kenya's finance minister said there was no need for international mediation.

Samuel Kivuitu, head of Kenya's election commission, has told the BBC he could not say for sure if Mr Kibaki had won the poll fairly.