LONDON (AFP) - When a cyclist is killed on London's roads, a flower-laden white bicycle is sometimes placed at the scene - both as a tribute to a lost friend and a warning to others.

More of these poignant "ghost bikes" may appear for the six cyclists dragged under lorries and buses in two weeks - a sudden spate of deaths that has shocked Londoners and raised tough questions for authorities.

Mayor Boris Johnson loudly trumpets the benefits of cycling - it's cheap, good for the environment and helps people get fit - but critics say he could do more to keep them safe.

"Some of the cases that we've seen in the last few days really make your heart bleed," Johnson told London's LBC radio station this week.

The colourful Conservative - who is often spotted wheeling around the capital himself with a helmet perched on his shaggy blonde head - refused to engage in any "blame or finger-pointing".

"But unless people obey the laws of the road," he added, "there's no amount of traffic engineering that we invest in that is going to save people's lives."

The deaths have sparked a war of words between cyclists who blame the city's poor infrastructure and critics who say too many riders take foolish risks, including by jumping red lights.

Cyclists complain that London's shortage of segregated bike lanes forces them to dice with death by travelling alongside huge lorries, while the "cycle super-highways" sometimes amount to little more than strips of blue paint.

"It's just not very friendly out there," said Matt Renton, a 28-year-old product manager, one of thousands heading home on bikes in the evening rush hour despite the bitter cold.

"You go somewhere like Amsterdam, and they've got a completely different attitude towards cycling," added Renton, who was wearing a luminous yellow jacket to signal his presence to motorists.

"(In London) we're mostly just seen as a nuisance." At 14, the death toll for 2013 has drawn level with that of last year and is comparable with Amsterdam and Berlin, although far above the single fatality recorded in Paris since January.

But the recent spike in deaths has piled pressure on officials to act.

London police have tasked 2,500 officers with stopping badly-driven lorries and misbehaving cyclists, while Mayor Johnson hinted he would back a ban on people wearing headphones while on their bikes.

"Call me illiberal, but it makes me absolutely terrified to see them bowling along unable to hear the traffic," he said.

The mayor also faces renewed calls to consider banning heavy goods vehicles (HGVs) from the city centre during rush hours.

HGVs were involved in more than half of London's cycling deaths since 2010.

All six of the recent casualties, who included a 24-year-old Russian woman, were hit by lorries, buses or coaches.

The mayor's office announced in March that it was investing £1 billion (S$2 billion) in cycling infrastructure over the next decade, included segregated bike paths, but many say there is a more urgent need for change.

With journeys by bike up 66 percent over a decade, bicycles are now as common a sight on London's streets as its famous black cabs and red double-decker buses.

Their riders, often stereotyped as a brigade of Lycra-clad daredevils, are an increasingly vocal force.

Protesters from the Critical Mass pro-cycling movement sporadically hold up the traffic with their jubilant, hundreds-strong bike rides.

And the furore over a woman who tweeted about "bloody cyclists" suggests Britain's cyclists are becoming a force to be reckoned with.

Emma Way, 22, became a target of an Internet hate campaign after she gloated on her Twitter page in May that she had "definitely knocked a cyclist off his bike" on a country lane.

The resulting storm of angry tweets attracted the attention of the police, and she was convicted this week of failing to stop after the collision or to report the accident.

Way, who received a fine for the offences, described the tweet as "the biggest regret of my life".