Billionaire Tom Perkins has apologized for comparing a row about free buses for tech workers in Silicon Valley to the persecution of the Jews in Nazi Germany.

It follows a month of protests in San Francisco over what some residents see as the negative impact of tech workers.

In an open letter to the Wall Street Journal, Tom Perkins described a “rising tide of hatred” of the rich.

His comments were criticized on Twitter.

In the letter, Tom Perkins said: “I would call attention to the parallels of fascist Nazi Germany to its war on its <<1%>>, namely its Jews, to the progressive war on the American 1%, namely the <<rich>>.

“This is a very dangerous drift in our American thinking. Kristallnacht was unthinkable in 1930; is its descendant “progressive” radicalism unthinkable now?”

He later said he regretted using the word Kristallnacht but his message about a new type of class warfare remained true.

Tom Perkins has apologized for comparing a row about free buses for tech workers in Silicon Valley to the persecution of the Jews in Nazi Germany

Kristallnacht – also referred to as night of broken glass – was a series of attacks against Jews in November 1938.

Tom Perkins, who headed up venture capitalist firm Kleiner Perkins Caufield & Byers before his retirement, was condemned by his old firm, which tweeted: “We were shocked by his views… and do not agree.”

The protests over the way tech workers commute to their jobs began in December when a bus taking people to Google’s campus had its window smashed by activists.

Similar protests have been held at bus stops around the city, with protesters complaining an influx of rich technology workers is driving up costs in the city.

The buses have become a symbol of such gentrification.

“Big tech exploits San Francisco’s cultural diversity and public infrastructure to lure workers here,” said the Heart of the City collective, which has organized the protests, in its own open letter also published this week.

“Real estate speculators capitalize on the influx of high-wage earners by evicting long-time residents to rent units at inflated rates, commanding up to 20% more around tech shuttle stops,” it added.

It is demanding that tech companies fund affordable housing initiatives and public transit service improvements.

The tech firms based in Silicon Valley, including Google, Twitter and Apple, use the buses to take about 17,000 people to and from the area each day.

Those behind the scheme say such buses ease congestion on already clogged roads in the city.

To ease the tensions, San Francisco’s transport agency has imposed fees and restrictions on the shuttle buses. The bus operators will have to pay $1 per stop per shuttle, netting an average of between $80,000 and $100,000 per operator each year.