SAN FRANCISCO — Firefighters, determined to keep a massive blaze from raging into the heart of California’s Yosemite National Park, have shut down half of its main east-west corridor, just days before a holiday weekend, marking the end of the peak summer tourist season.

A long stretch of Tioga Road through the western half of the park was closed on Wednesday, Aug. 28, as fire crews tightened their grip on the blaze, extending containment lines around 30 percent of the wildfire’s perimeter by the end of its 12th day.

Firefighters work to prevent the Rim Fire from jumping Highway 120 near Buck Meadows, California, August 24, 2013. REUTERS/Max Whittaker

The so-called Rim Fire has blackened about 192,500 acres, mostly in the Stanislaus National Forest, west of the park. Its cause is still being investigated.

With an overall footprint that exceeds the land mass of Chicago, the blaze ranks as the sixth largest California wildfire on record.

The firefighting force of more than 4,500 is backed by teams of bulldozers, water-dropping helicopters and airplane tankers, carrying payloads of flame-retardant chemicals.

The battle inside Yosemite is focused largely on preventing flames from advancing any farther toward the core of the park, 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.