Teachers in the borough are joining others around the country on a strike over pay, pensions and workload.
Members of Harrow's National Union of Teachers (NUT) are joining a mass rally in central London today, which has led to the closure of seven schools and partially closed 23 others.
The strike is over government changes to education, which the union says are having a damaging effect on education.
Simon Sackwild, president of Harrow NUT, said: “We are going on strike today because we have been backed into a corner because the government do not want to have meaningful talks. We are taking action to defend education.
“It is always a tough decision to go on strike but we have to show we are concerned about the changes being brought in.
“Performance related pay is a big concern because results do not show how well a teacher is doing.
“All teachers work hard and there is no evidence that payment by results improves standards.
“The Government wants to improve standards in education but we haven’t had an increase in pay for years.”
Other changes include teachers having to work until they are 68 or beyond, increasing pensions contributions by 50 per cent and the introduction of performance-related pay.
Harrow Borough Council leader Cllr Susan Hall, said: “I obviously regret that this strike is going ahead.
“I know teachers feel strongly about the issues involved, but we all ultimately want what is best for our children and I hope we can get to an agreement without the need for any further disruption.”
London NUT regional secretary Bob Stapley said: “It is time for the Secretary of State seriously to address the issue of teacher workload if we are to retain the excellent teacher we have in London schools.
“Teachers do not take strike action lightly but they believe that nothing else will bring about the meaningful negotiations we are seeking.”
A spokesman for the Department for Education said: “Parents will struggle to understand why the NUT is pressing ahead with strikes over the Government's measures to let heads pay good teachers more.
"They called for talks to avoid industrial action, we agreed to their request, and talks have been taking place weekly.
"Despite this constructive engagement with their concerns, the NUT is taking action that will disrupt parents' lives, hold back children's education and damage the reputation of the profession.”