Three guards and one police officer were killed, as were three militants, but two are still fighting, according to the chief of Kabul police.
An unknown number of people have been taken hostage, including women and children, but 18 have been freed.
The Taliban claimed the attack. Lake Qargha is on the outskirts of Kabul and is favored by residents of the Afghan capital for day trips and family outings.
The gunmen, armed with rocket-propelled grenades and heavy machine guns, launched the attack on the hotel late on Thursday, officials say.
“Police forces have besieged the hotel where people gathered for a picnic or wedding party,” Kabul deputy police chief Daoud Amin told the AFP news agency.
“We will be very careful so that not to harm to civilians,” he added.
Some guests reportedly jumped out of hotel windows to flee from the attackers.
An unknown number remain inside.
The US-led coalition is providing support to the Afghan security forces, who are trying to flush out the insurgents from the hotel.
Reuters news agency quotes the Taliban as saying they attacked the hotel because wealthy Afghans and foreigners used it for “wild parties”.
Violence has recently increased across the country, with at least three US soldiers and about 20 Afghans killed in a series of attacks over the past seven days.
The attacks come as NATO prepares to hand responsibility to Afghan forces when the alliance’s combat troops leave in 2014.
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