KANDAHAR, Afghanistan – A suspected Al Qaeda suicide bomber walked into a mosque during the funeral of a Muslim cleric and blew himself up yesterday, killing 20 people, including Kabul’s police chief, and wounding 42 others.

The attack was the deadliest in Afghanistan since a surge in violence began in March, casting doubt on American claims that it is stabilizing the country and reinforcing fears that militants here are copying the tactics of those in Iraq.

Hundreds of mourners were crowded inside the mosque for the funeral of Mullah Abdul Fayaz in the main southern city of Kandahar when the bomber struck.

President Karzai condemned the assault as an “act of cowardice by the enemies of Islam and the enemies of the peace of Afghan people” and ordered a high-level investigation.

Parts of the bomber’s body were found, and Kandahar Governor Gul Agha Sherzai said he belonged to Osama bin Laden’s terrorist network.

“The attacker was a member of Al Qaeda. We have found documents on his body that show he was an Arab,” Mr. Sherzai said. “We had an intelligence report that Arab Al Qaeda teams had entered Afghanistan and had been planning terrorist attacks.”

A purported Taliban spokesman, Mullah Latif Hakimi, said in a telephone call to the Associated Press that the rebels were not responsible for the bombing. Mr. Hakimi often calls news organizations, usually to claim responsibility for attacks on behalf of the former regime. His information has sometimes proven untrue or exaggerated, and his exact tie to the rebel leadership is not clear.

Among those killed yesterday was the Kabul police chief, General Akram Khakrezwal, two of his nephews, and six of his bodyguards, Mr. Sherzai said. The attacker detonated the explosives after coming close to the police commander, but it was not clear if he was targeted, an Interior Ministry spokesman, Latfullah Mashal, said.

Khakrezwal, a supporter of Mr. Karzai, was police chief in the capital for two months. Prior to that, he had been police commander in a northern city and for Kandahar.

Many local leaders had been expected to attend the funeral of Fayaz, the top Muslim leader in the province.

Fayaz, also a supporter of Mr. Karzai, was shot to death in Kandahar on Sunday by suspected Taliban gunmen – a week after he led a call for people not to support the rebels.

Kandahar was a stronghold of the Taliban regime that was ousted from power in late 2001 by American-led forces for harboring Mr. bin Laden.

The American military spokesman in Afghanistan, Colonel James Yonts, condemned the blast, calling it an “atrocious act of violence upon innocent civilians and a mosque.”

While Mr. Sherzai put the casualty toll at 20 dead and 42 wounded, the director of the Kandahar Hospital, Mohammed Hashim Alokozai, said 72 were injured, four gravely.

The deputy police chief, General Salim Khan, said the explosion occurred near where people remove their shoes before praying.

“I was knocked unconscious by the blast. When I woke up, so many people were killed or wounded. People were running around, some were lying on the ground crying. Dead bodies were everywhere,” a mourner who was behind a wall in the mosque when the bomb exploded, Nanai Agha, said.

In a second attack yesterday, a bomb exploded on a bridge west of Kandahar as a group of Afghan explosives experts working on a Japanese funded demining project were driving over it, killing two of them and wounding five, a spokesman for the U.N. Mine Action Center for Afghanistan, Patrick Fruchet, said.

Although the insurgents have stepped up their offensive, they have also suffered heavy casualties, losing about 200 men since March, following a winter lull in the fighting, according to American and Afghan officials.

Tension has been high in Afghanistan and neighboring Pakistan after deadly anti-American riots sparked by a news report – later retracted – that interrogators at Guantanamo Bay, Cuba, mistreated the Koran.