NAIROBI, Kenya — A grenade attack on a political rally in downtown Nairobi on Sunday evening killed six people and injured more than 100, cranking up tensions ahead of a hotly contested referendum on a new constitution.

Three grenades exploded one after the other at dusk in Uhuru Park, a treed carpet of green that stretches the length Nairobi’s central business district. The explosions caused a panicked stampede among thousands of Kenyans who had gathered to campaign for a ‘No’ vote in the forthcoming constitutional referendum due in early August.

The injured were taken to Kenyatta National Hospital where the luckier ones were laid on gurneys while others, bloodied and battered, littered the floor of the overstretched emergency department.

The blood, death and fear of Sunday evening were an unwelcome flashback for a country still feeling the aftershocks of political violence that killed more than 1,100 people following the last elections. Not a single person has yet been convicted although an investigation underway by the International Criminal Court (ICC) may change that.

Political violence is nothing new to Kenya. Since multi-party democracy began in 1991 politics and violence have walked hand-in-hand. Tensions rise before elections often erupting in ethnic attacks stoked by politicians who emphasize tribal allegiances and stoke local animosities.

The run-up to the constitutional referendum on Aug. 4 has been hotly contested, as it was in 2005 when a proposed new constitution was voted down, setting the stage for the disputed and ultimate bloody election of late 2007.

On Tuesday six politicians were called in for questioning by Kenya’s National Cohesion and Integration Commission for alleged, “hate speech” during the campaign.

A new constitution is widely considered to be long overdue. The current one dates back to independence in 1963 and since the early 1990s a replacement has been debated and even fought over. The proposed new constitution trims the president’s sweeping powers, devolves more authority to the regions and addresses long-standing land grievances often seen as being at the root of Kenya’s strife.

But it also allows abortion, “if the life or health of the mother is in danger,” and special Muslim courts for “matters relating to personal status, marriage, divorce and inheritance.”

These latter clauses have upset Christian groups so Sunday’s rally was a politico-religious affair with politicians and preachers standing side-by-side to condemn the draft law.

As the dust settled in Uhuru Park and emergency doctors tended to the injured, accusations quickly began to fly. ‘No’ and ‘yes’ campaigners blamed one another. The ‘no’ campaigners said it was an attempt to frighten their supporters, the ‘yes’ campaigners that it was an orchestrated attack designed to win sympathy. Church groups, like the ‘no’ campaigners, blamed the government for the attack.

“Having been informed over and over that the passage of the new constitution during the referendum is a government project, we are left in no doubt that the government, either directly or indirectly, had a hand in this attack,” the National Council of Churches of Kenya said in a statement. “Who else in this country holds explosive devices?”

President Mwai Kibaki and Prime Minister Raila Odinga support the draft constitution, so do the majority of Kenyans according to opinion polls, but the Christian lobby is powerful, as are some of the politicians arrayed against them.

These include William Ruto, higher education minister, regarded as the flagbearer of the ‘no’ campaign. He is among the six called in for “hate speech” this week and is one of 19 politicians named in a public list of alleged perpetrators of the post-election violence.

Analysts say that many powerful politicians have a vested interest in maintaining the status quo or seeking to disrupt the planned referendum. Even before Sunday’s attacks there was evidence that some were willing to employ dirty tricks to undermine efforts at reform.

An early draft of the constitution went to the government printer only to emerge with a reference to “national security” inserted into the new Bill of Rights section. It was a ham-fisted attempt to undermine the very rights that the new constitution is designed to enshrine. A police investigation into who was responsible has led nowhere.

Senior government officials have called for calm and for Kenyans to wait for the result of the police investigation but there is so little faith in the police that people here are busily speculating over who

was really behind Sunday’s attack.

Macharia Gaitho, a commentator for the Daily Nation newspaper, expressed the opinion of many when he wrote: “If [the police] do not catch culprits within a reasonable time … I will have a license to draw my own conclusions based on who benefits from the murderous actions.”