The officers will be working overtime, which SEPTA will pay for, Blackburn said.
City patrol officers have been put on alert and will be making scheduled checks on buses and at stations, Blackburn said.
SEPTA Police Chief Richard Evans said the security firm AlliedBarton would provide 40 guards to staff certain locations. Twenty will work a 6 a.m.-to-2 p.m. shift. The other 20 will work from 2 to 10 p.m.
About 45 supervisory officers who are not on strike will work extended shifts as needed, Evans said.
Richard Neal Jr., union president, said 150 to 170 transit officers typically are on duty between 2 and 4 p.m. weekdays.
Neal questioned the purpose of the security guards, noting that they do not have the power to arrest people.
"The math doesn't add up," said union spokesman Anthony Ingargiola, suggesting that the riding public would be less safe while the transit officers were on strike.
Ingargiola said the union, which has 219 members, had provided SEPTA a "last best offer" in February that included many concessions.
He said the only sticking point was a union demand for a 50-cent-an-hour pay raise based on the officers' police certification, which has to be maintained through regular training updates, that would cost SEPTA about $200,000 a year.
The two sides already had agreed to an 11.5 percent general pay increase over the five years of the next contract, Ingargiola said.
The two sides met Wednesday, and the agency came back with an offer of 15 cents an hour, Ingargiola said.
The union "got slapped in the face," Ingargiola said.
Shortly afterward, the union decided to go on strike.
SEPTA spokesman Richard Maloney declined to discuss details of the negotiations, but said that what the union was demanding was out of line with what the agency had agreed to with its other unions.
He called the strike "totally unnecessary" and complained that "we were given 20 minutes' notice" that the officers were walking off the job.
Ingargiola retorted: "The notion that SEPTA was caught off guard is balderdash."
Neal said he had been in touch with U.S. Rep. Bob Brady (D., Pa.) to possibly broker a deal.
In 2008, the transit police union, which then had been without a contract for 32 months, went on strike on a Friday afternoon and returned to duty the next morning after Mayor Nutter prodded both sides back to the bargaining table. A few days later, a deal was reached.
If there is no quick resolution this time around, union members say, they are willing to stand their ground.
"I think the officers are willing to be out here for as long as it takes," said Jorge Alicea, 30, who has been on the force for more than three years.
"We're just like any other police officers," he said.
At Suburban Station, commuter Doris Brown, 56, of Mount Airy, said she would feel safer with the transit officers back on the job.
"I think they're really needed," she said.
Contact staff writer Robert Moran at 215-854-5983 or firstname.lastname@example.org, or follow @RobertMoran215 on Twitter.
Inquirer staff writer Sam Wood contributed to this article.