The FTSE 100 <.FTSE> index ended the session down 0.9 percent at 7,323.98 points, extending the previous session's losses after three days of gains. It still managed to end the week with a small gain, however.

A suspected Islamist militant drove a van into crowds in Barcelona on Thursday, killing 13 people and injuring more than 100.

Shares in International Consolidated Airlines (>> International Consolidated Airlines Group), which operates British Airways, and easyJet (>> EasyJet) fell 2 percent and 0.8 percent, recovering earlier losses slightly as the broader European travel & leisure sector <.SXTP> dropped 1.5 percent.

"The entire airline and leisure industry is down today purely from the attacks," said John Moore, trader at Berkeley Capital.

"People in the forthcoming months, we believe, will be less likely to take trips abroad," he said, adding that he expected the airline stocks to recover.

Randgold Resources (>> Randgold Resources) led the 10 or so risers, up 1.2 percent as investors sought safety in assets such as precious metals miners and the price of gold rose for the third day straight on the back of political turmoil in the United States.

Falls among large consumer staples firms such as British American Tobacco (>> British American Tobacco) and Diageo (>> Diageo) were the biggest weights on the FTSE 100, while financials also added pressure with HSBC (>> HSBC Holdings), Lloyds (>> Lloyds Banking Group) and Barclays (>> Barclays) edging lower.

Energy stocks took 6.6 points off the index, with BP (>> BP) declining 0.7 percent as the price of oil fell. [O/R]

British mid caps <.FTMC> were also 0.7 percent lower, with Hikma (>> Hikma Pharmaceuticals) among the biggest fallers, extending losses into a second day after trimming its sales forecasts on Thursday.

The health care firm has fallen around 15.5 percent over the past two sessions, and broker HSBC cut its target price on the stock.

"Management also repeatedly stated that pricing is cyclical, the industry has been here before and that it will recover. We disagree," analysts at HSBC said in a note, adding that they expect U.S. drug pricing to remain under increasing pressure through the second half of 2017 and onwards.

(Reporting by Kit Rees; editing by Andrew Roche and Toby Davis)

By Kit Rees