In the fog-drenched hours of Saturday morning, several blocks of downtown San Francisco became the scene of a massive terrorist invasion of the Bay Area’s financial epicenter. At least, that’s the way it felt. Nearly five blocks surrounding California and Montgomery streets were closed to traffic for much of the day as part of a massive terrorist drill costing $250,000 in Homeland Security grants. Hundreds of police, fire and military personnel responded to the incident, helping to defuse a bomb, track down suspects and rescue victims played by student volunteers. The drill included three separate attacks — the first two in the Bank of America building at 555 California St. and the third at 1 Post St. Nearly 20 agencies responded to the incidents and were evaluated on their performance in defusing the crises.