(Adds latest acreage figures, number of firefighters, weather forecasts)
By Laila Kearney
SAN FRANCISCO, Aug 29 (Reuters) - Firefighters determined to keep a massive blaze from invading the heart of California's Yosemite National Park took advantage of cooler weather early on Thursday to slow the spread of flames ahead of a holiday weekend marking the end of the peak summer tourist season.
Progress came after a long stretch of Yosemite's main east-west road was closed on Wednesday through the western half of the park as crews tightened their grip on the blaze, extending containment lines around 30 percent of the fire's perimeter by the end of its 12th day.
Capping a week in which the footprint of the blaze grew by tens of thousands of acres, a cooling trend and rising humidity levels helped curb the fire's growth overnight, with just 270 acres added to the tally of charred landscape by dawn on Thursday.
Since erupting on Aug. 17, the so-called Rim Fire has blackened nearly 193,000 acres (78,104 hectares), or more than 300 square miles of dry scrub and timberlands, mostly in the Stanislaus National Forest west of the park, fire officials said on Thursday. Its cause was still being investigated.
The firefighting force has grown to more than 4,800 personnel, consisting mostly of crews wielding hand tools, chain saws and even special torches to create fire breaks by clearing the rugged terrain of unburned trees and chaparral ahead of advancing flames.
They are backed by teams of bulldozers, water-dropping helicopters and airplane tankers carrying payloads of flame-retardant chemicals.
The battle inside Yosemite was focused largely on preventing flames from penetrating any farther toward the core of the park, including the Yosemite Valley area famed for its towering rock formations, waterfalls, meadows and pine forests.
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August. Park officials said they have already noticed a decline in the crowds of visitors typically seen in late summer.
With an overall footprint that exceeds the land mass of The city of Chicago, the blaze ranks as the sixth-largest California wildfire on record.
As of Wednesday, less than a quarter of the total burned landscape, about 43,000 acres, lay inside Yosemite, confined to the northwest corner of the 750,000-acre (300,000-hectare) park.
Earlier this week, flames closed in on the park's Hetch Hetchy Reservoir, which supplies 85 percent of the water consumed by San Francisco and several other communities some 200 miles (320 km) to the west.
But utility officials said there was little risk to the artificial lake because of the rocky terrain and lack of brush surrounding it.
On the opposite end of the sprawling fire zone west of Yosemite, crews fought to keep flames away from some 4,500 homes in a string of small communities along the fringe of the Stanislaus National Forest, said California Department of Forestry and Fire Protection spokesman Dennis Matheson.
Most of those dwellings have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment's notice. The fire has already destroyed dozens of homes and cabins, but no serious injuries have been reported.
Matheson said treacherous, hard-to-reach terrain was hindering efforts to carve additional containment lines around the blaze, and he estimated it would take another week to fully enclose it.
"I think it's very safe to say that we're looking at least at the first week of September," Matheson said.
The flames last week forced the closure of a stretch of Highway 120 that leads into the west side of the park and serves as the main gateway from the San Francisco Bay area.
On Wednesday, the closure was extended eastward along 120, also called Tioga Road, as far as Yosemite Creek midway through the park. But the rest of the road through Yosemite's only east entrance remained open, along with two other gates that allow access from other directions in the southern half of the park.
The expanded closure of Highway 120 - the only east-west route that completely bisects the park - was imposed to allow fire crews to establish new containment lines along the road before the blaze approaches, said Yosemite spokesman Tom Medema.
"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and (be)an inconvenience for visitors," he said.
Several campgrounds and trails, along with two landmark groves of giant sequoia trees, also were closed to the public.
Firefighters on Thursday also continued to burn containment lines from the Hetch Hetchy Reservoir south to Tioga Road in a bid to keep the fire from creeping further east into the park, Medema said.
The blaze has been among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West in recent weeks, straining national firefighting resources. (Writing by Steve Gorman; Editing by Mark Trevelyan)